Origins
The suffix '-ful' is a native Germanic adjective-forming suffix in English, historically identical with the adjective 'full.' It attaches to nouns (and occasionally verbs) to form adjectives meaning 'full of,' 'characterised by,' or 'tending to': 'beauty > beautiful,' 'care > careful,' 'joy > joyful,' 'use > useful,' 'hope > hopeful,' 'help > helpful,' 'wonder > wonderful.'
The suffix descends from Old English '-ful' or '-full,' which was simply the adjective 'full' (from Proto-Germanic *fullaz, full) used in compound adjectives. Proto-Germanic *fullaz itself descends from Proto-Indo-European *pleh₁- or *pelh₁-, meaning 'to fill.' This is one of the deepest and most widely attested PIE roots, producing Latin 'plēnus' (full — whence 'plenty,' 'plenary,' 'plenitude,' 'plenipotentiary,' 'replete'), Greek 'pleos' (full, whence 'pleroma'), Sanskrit 'pūrṇa' (full), Old Irish 'lán' (full), and native English 'fill,' 'full,' 'folk' (originally 'fullness of people'), 'flock,' and 'fulfil.'
In Old English, '-ful' appears in many compound adjectives: 'blōdful' (bloody, full of blood), 'cārful' (careful, full of care), 'ēstful' (gracious, full of favour), 'grīmful' (grim, fierce), 'hoganful' (careful), 'mildful' (mild, full of mildness — rare), 'sorhful' (sorrowful), 'sūnful' (sinful), 'þancful' (thankful), 'woruldful' (worldly), 'wrætful' (wrathful). The pattern is consistent: noun + 'full' = adjective meaning 'full of that quality.'
Middle English
In Middle English (c. 1150–1500), '-ful' remained fully productive and began to attach to borrowed French and Latin nouns as well: 'beautiful' (beauty + ful, first attested 15th c., beauty itself from Old French 'biaute'), 'graceful' (grace + ful, 1586), 'merciful' (mercy + ful, c. 1225), 'peaceful,' 'pitiful,' 'plentiful,' 'respectful' (17th c.), 'shameful,' 'successful' (1590s), 'thankful,' 'truthful,' 'useful' (1590s), 'wasteful,' 'wishful,' 'wonderful' (1150, from wonder + full), 'youthful.' By the seventeenth century, '-ful' was attaching to Latinate abstractions as freely as to native bases.
The suffix remains one of the dozen most productive in modern English. Speakers can coin nonce '-ful' adjectives with ease: 'eventful,' 'flavourful,' 'meaningful,' 'purposeful,' 'resourceful,' 'respectful,' 'skilful,' 'spiteful,' 'stressful,' 'thoughtful,' 'tuneful,' 'watchful.' Newer coinages are occasionally awkward but generally understandable: 'angst-ful,' 'tech-ful' (rare), 'snark-ful' (rare).
A secondary use of '-ful' is as a noun-forming suffix meaning 'the quantity that fills the base.' This gives: 'cupful' (the amount a cup holds), 'handful,' 'mouthful,' 'spoonful,' 'bucketful,' 'basketful,' 'bowlful,' 'boxful,' 'carful,' 'carload (different suffix),' 'eyeful,' 'fistful,' 'glassful,' 'houseful,' 'jarful,' 'lungful,' 'plateful,' 'pocketful,' 'potful,' 'roomful,' 'roomful,' 'shovelful,' 'stomachful.' These are concrete count nouns, plural with '-s' in the usual way. The plural of 'handful' is conventionally 'handfuls' rather than 'handsful,' reflecting its treatment as a single unit rather than a compound.
Modern Usage
Spelling rules for '-ful' are straightforward. The suffix always has one 'l,' even though the base word 'full' has two. This is a late spelling convention — up until the eighteenth century '-full' was acceptable and indeed common. Modern English retains the double 'l' only in the free word 'full' and in compounds where the second element is 'full' as a separate word (e.g., 'chock-full').
Final '-y' in a base becomes '-i-' before '-ful' ('mercy > merciful,' 'duty > dutiful,' 'plenty > plentiful,' 'beauty > beautiful' — 'beauti-' being the stem). Final '-e' is kept in some cases: 'hope + ful = hopeful.' Compound bases attach the suffix to the whole compound: 'daydream > daydream-ful' (nonce).
Phonologically, '-ful' is pronounced /fəl/ or /fʊl/ with an unstressed vowel; stress stays on the base. Because the suffix is native, the pronunciation is fully integrated and does not alter vowel quality in the base ('beauty' > 'beautiful,' same /juː/ in both).
Later History
The semantic range of '-ful' is narrower than it first appears. The core meaning is 'full of the quality denoted by the noun,' but the adjective often drifts from this literal sense. 'Awful' originally meant 'full of awe' (inspiring awe), then 'extremely impressive,' then 'extremely bad' — a complete semantic reversal. 'Wonderful' originally meant 'full of wonder' (marvellous) and now often means simply 'very good,' with the wonder part weakened. 'Dreadful' (full of dread) and 'fearful' (full of fear) have similarly drifted from genuine dread or fear to mild negative evaluation.
The suffix '-ful' has a natural antonym in the equally native '-less' (meaning 'without'). Many roots form both: 'careful / careless,' 'harmful / harmless,' 'helpful / helpless,' 'hopeful / hopeless,' 'meaningful / meaningless,' 'painful / painless,' 'successful / unsuccessful,' 'tactful / tactless,' 'thankful / thankless,' 'thoughtful / thoughtless,' 'useful / useless.' This pair is one of the cleanest examples of antonym-forming in English morphology.
Less common is the doublet '-ful' / '-y.' Many adjectives can be formed either way from a noun: 'sun > sunny' (most common) versus 'sun > sunful' (rare/nonce), 'wind > windy' versus 'wind > windful' (rare), 'storm > stormy' versus 'storm > stormful' (rare). In general '-y' is preferred for adjectives based on weather or short concrete nouns, while '-ful' is preferred for abstract nouns.
Legacy
Representative '-ful' adjectives include: artful, awful (originally ''full of awe,'' now usually negative), bashful, beautiful, blissful, bountiful, careful, cheerful, colourful, deceitful, delightful, disdainful, disgraceful, disrespectful, doubtful, doleful, dreadful, dutiful, eventful, fateful, faithful, fanciful, fearful, fitful, flavourful, forceful, forgetful, fretful, fruitful, gainful, gleeful, graceful, grateful, handful (noun), harmful, hateful, healthful, heedful, helpful, hopeful, hurtful, insightful, joyful, lawful, masterful, meaningful, merciful, mindful, mirthful, mournful, neglectful, painful, peaceful, pitiful, plentiful, playful, powerful, prayerful, prideful, purposeful, regretful, remorseful, resentful, resourceful, respectful, restful, revengeful, rightful, sinful, skilful, sorrowful, soulful, spiteful, stressful, successful, tactful, tasteful, thankful, thoughtful, tearful, truthful, tuneful, unfaithful, unhelpful, unhopeful, unmerciful, unsuccessful, ungrateful, unhelpful, uneventful, ungodful (rare), unlawful, unmindful, unremorseful, unrespectful, unskilful, unthoughtful, unthankful, unuseful, unwatchful, useful, vengeful, watchful, wasteful, wilful, wishful, woeful, wonderful, worshipful, wrathful, wrongful, youthful, zestful. Representative '-ful' nouns (quantity): bagful, barrelful, basinful, basketful, bowlful, boxful, bucketful, carful, cartful, cupful, dishful, eyeful, fistful, glassful, handful, hatful, houseful, jarful, ladleful, lapful, mouthful, netful, pailful, plateful, pocketful, potful, roomful, sackful, shelfful, shovelful, spoonful, stomachful, tableful, truckful, trunkful.