-ly

The suffix '-ly' is one of the most productive and familiar suffixes in English, used to form adverb‌​‍​‍​‌​‌​‍​‍​‍​‌​‍​‌​‍​‍​‍​‌​‍​‍​‌​‍​‌​‌​‌​‍​‌​‍​‍​‌​‌​‌​‍​‍​‍s from adjectives (quickly, softly, carefully) and adjectives from nouns (manly, friendly, heavenly). It has a remarkable etymology: it descends from a free-standing Proto-Germanic noun meaning 'body.'

The suffix comes from Old English '-līċ' (adjective-forming, as in 'manlīċ,' manly) and '-līċe' (adverb-forming, as in 'cwiclīċe,' quickly). Both descend from Proto-Germanic *-līkaz, itself derived from the noun *līką meaning 'body, form, likeness.' In other words, the ancestor of '-ly' was a full word meaning 'body,' and the suffix originally meant 'having the body of' or 'in the form of.' 'Manly' is etymologically 'man-bodied'; 'quickly' is 'in the manner / form / body of quick.'

The Old English noun 'līċ' (body, corpse) survives today in the compound 'lich-gate' or 'lych-gate' (the covered gateway of a churchyard where bodies were historically rested before burial). Cognates show the same history: German 'Leiche' (corpse), Dutch 'lijk' (corpse), Swedish 'lik.' The adjective-forming suffix developed from this noun: '-bodied' > '-shaped' > '-having the form of' > the current sense of resemblance.

Modern English 'like' is the direct free-standing descendant of the same root. 'Man-like' and 'manly' are doublets — the first a compound using the free-standing word, the second a historical suffix form. When English speakers coin new '-like' words ('bird-like,' 'dream-like') they are unconsciously reusing the same root that gives 'likely,' 'manly,' and 'friendly.' This makes '-like' and '-ly' essentially siblings: the same morpheme, one free, one bound.

In Old English the two forms were distinct: '-līċ' formed adjectives ('friend' + '-līċ' = 'frēondlīċ,' friendly), while '-līċe' formed adverbs ('cwic' + '-līċe' = 'cwiclīċe,' quickly). The final '-e' of the adverbial form was dropped in late Middle English (c. 1400s), collapsing the two into a single spelling '-ly.' Modern English distinguishes adjectives and adverbs in '-ly' purely by syntactic context: 'a friendly wave' (adjective) versus 'waved friendly-ly' (awkward because the adverb is the same spelling; modern speakers prefer 'in a friendly manner').

From Middle English onward, '-ly' became the dominant adverb-forming suffix in English. Adverbs are formed by adding '-ly' to adjectives: 'quick > quickly,' 'soft > softly,' 'careful > carefully,' 'happy > happily' (with spelling change y > i), 'complete > completely,' 'real > really,' 'true > truly' (dropping final e). This pattern is fully productive: any adjective can form an adverb in '-ly,' including Latinate borrowings ('absolutely,' 'completely,' 'normally,' 'usually') and novel coinages ('digitally,' 'virtually,' 'biometrically').

The adjective-forming '-ly' survives in a smaller, closed set of words, mostly quite old: 'bodily,' 'costly,' 'deadly,' 'earthly,' 'friendly,' 'ghostly,' 'godly,' 'heavenly,' 'homely,' 'kindly,' 'lively,' 'lonely,' 'lovely,' 'manly,' 'motherly,' 'nightly,' 'princely,' 'saintly,' 'worldly.' These are adjectives, not adverbs, and they look exactly like '-ly' adverbs — which can cause confusion. The sentence 'He is friendly' uses 'friendly' as an adjective; you cannot readily say 'He spoke friendly' as an adverb, because '-ly' cannot be stacked on '-ly' (no 'friendly-ly'). Instead, speakers rephrase ('He spoke in a friendly manner').

A handful of words have both adjective and adverb forms in '-ly': 'daily,' 'weekly,' 'monthly,' 'yearly' can all be adjectives ('a daily newspaper') or adverbs ('he visits daily'). These are exceptional cases where the same form fulfils both functions, and they are the closest English has to a single '-ly' suffix serving both roles.

Occasionally '-ly' attaches to numerals ('early,' from Old English 'ǣr' + '-līċe') or nouns ('timely,' 'stately,' 'orderly'). These are historical formations and new ones rarely occur.

A noteworthy feature of '-ly' is that it generally does not attach to adjectives already ending in '-ly' (no 'friendly-ly,' 'silly-ly,' 'likely-ly'). Instead, speakers use periphrastic constructions ('in a friendly way,' 'in a silly manner'). It also sometimes alters spelling: 'happy > happily' (y to i), 'true > truly' (dropping final e), 'whole > wholly' (doubling l), 'due > duly' (dropping final e).

Representative '-ly' adverbs (from adjectives): absolutely, actually, angrily, badly, beautifully, carefully, certainly, clearly, completely, easily, exactly, extremely, finally, fully, generally, gently, happily, hardly, honestly, hopefully, immediately, likely, loudly, mostly, naturally, normally, obviously, often (etymologically different), only, perfectly, possibly, probably, quickly, quietly, rarely, really, recently, regularly, safely, seriously, simply, slowly, softly, suddenly, surely, totally, truly, typically, usually, very (from Latin, not -ly), wholly. Representative '-ly' adjectives (from nouns): bodily, costly, cowardly, daily, deadly, earthly, fatherly, friendly, ghostly, godly, heavenly, homely, hourly, kingly, knightly, lovely, manly, monthly, motherly, nightly, orderly, princely, scholarly, timely, weekly, worldly, yearly.