light

The adjective 'light' meaning 'not heavy' is one of the most instructive examples in English etymology of two completely unrelated words colliding into a single form.‌​‍​‌​‍​‍​‍​‍​‍​‌​‌​‍​‍​‌​‍​‌​‌​‌​‌​‌​‌​‍​‍​‍​‌​‌​‍​‌​‍​‌​‍​‌​‍ The brightness 'light' and the weightlessness 'light' do not share an ancestor — they descend from separate Proto-Indo-European roots that happened, through independent sound changes operating across a thousand years of English phonology, to produce identical results. Linguists call this kind of collision 'homophony by convergence,' and English is unusually rich in such accidents.

The 'not heavy' sense descends from Old English 'lēoht' (also spelled 'līht'), from Proto-Germanic *linhtaz, from PIE *h₁lengʷh-, a root reconstructed to mean 'light in weight, agile, nimble.' The laryngeal *h₁ in the PIE reconstruction indicates an initial consonant in the ancestral language that left no direct trace in most daughter languages but influenced the quality of adjacent vowels. The root *h₁lengʷh- is attested with reasonable confidence across multiple branches.

In Latin, the same root produced 'levis' (light in weight, slight, trivial), which gave English an enormous secondary vocabulary. 'Levity' (lightness of manner), 'levitate' (to rise as if weightless), 'lever' (a mechanical device exploiting a pivot — the name refers to the lifting or lightening of a load), 'elevate' (to raise up), and 'alleviate' (to lighten a burden) all derive from Latin 'levis.' 'Relief' comes from Old French 'relever' (to lift again, to lighten), from the same root. Even 'carnival' carries a trace of it: from Latin 'carnem levare' (to remove or lighten the meat — the feasting before the meatless weeks of Lent).

In Sanskrit, the cognate is 'laghu' (light, swift, small), used in classical literature to describe anything agile or unencumbered. In Greek, the cognate 'elakhýs' (small, light) is less prominent but attested. Old Norse had 'létt,' and Old High German had 'lihti,' both meaning light in weight — ancestors of German 'leicht' (easy, light) and Dutch 'licht' (light, slight).

The phonological history of the English form is interesting. Old English 'lēoht' had a back vowel where one might expect a front vowel, and the 'h' at the end represented a velar fricative (the sound in Scottish 'loch'). Through Middle English, this velar fricative was lost in most dialects, leaving a lengthened vowel that subsequently shifted through the Great Vowel Shift to produce the modern diphthong /aɪ/. The result, purely by accident, was a form identical to the one produced independently by the Old English 'lēoht' (brightness) following its own sound change trajectory.

The brightness 'light' takes a completely different PIE path: *lewk- (to be bright, to shine), which produced Latin 'lux' (light), 'luna' (moon), 'lucidus' (clear, shining), and 'lucerna' (lamp), as well as Greek 'leukos' (white, bright), from which came 'leukemia' (white-blood disease). The two PIE roots — *h₁lengʷh- (weightlessness) and *lewk- (brightness) — have no connection, yet their English descendants are phonologically indistinguishable.

This convergence gave English poets a gift that speakers of other languages do not have. The double meaning of 'light' enabled wordplay impossible in German (where 'leicht' and 'Licht' remain distinct), in French (where 'léger' and 'lumière' are unambiguously separate), or in Latin (where 'levis' and 'lux' share no resemblance). Metaphors like 'light reading,' 'light-hearted,' and 'enlightenment' exploit the semantic overlap even where the etymological overlap is accidental.

The compound 'lightweight' records both senses in productive tension: in boxing and other combat sports, it refers strictly to weight; in everyday speech, 'a lightweight' means a person of no substance or consequence — an extension of the physical sense into moral judgment. 'Light-fingered' (prone to theft) plays on the nimble, agile quality of the PIE root, fingers that move so lightly they take what they shouldn't. 'Light-footed' similarly draws on the original idea of swift, agile movement, recalling the PIE sense of nimbleness alongside mere weightlessness.