Thin
Old English thynne, of Germanic origin; related to Dutch dun and German dünn, from an Indo-European root shared by Latin tenuis .
wiktionary
From Middle English thinne, thünne, thenne, from Old English þynne, from Proto-West Germanic *þunnī, from Proto-Germanic *þunnuz(“thin”) – compare *þanjaną(“to stretch, spread out”) – from Proto-Indo-European *ténh₂us(“thin”), from *ten-(“to stretch”).
Cognate with German dünn, Dutch dun, West Frisian tin, Icelandic þunnur, Danish tynd, Swedish tunn, Latin tenuis, Irish tanaí, Welsh tenau, Latvian tievs, Sanskrit तनु(tanú, “thin”), Persian تنگ (tang, “narrow”). Doublet of tenuis. Also related to tenuous.
etymonline
thin (adj.)
Old English þynne "narrow, lean, scanty, not dense; fluid, tenuous; weak, poor," from Proto-Germanic *thunni "thin" (source also of West Frisian ten, Middle Low German dunne, Middle Dutch dunne, Dutch dun, Old High German dunni, German dünn, Old Norse þunnr, Swedish tunn, Danish tynd), from PIE *tnu- "stretched, stretched out" (hence "thin"), from root *ten- "to stretch" (source also of Latin tenuis "thin, slender").
These our actors ... were all Spirits, and Are melted into Ayre, into thin Ayre. [Shakespeare, "The Tempest," IV.i.150, 1610]
"Loose or sparse," hence "easily seen through," with figurative extensions. Related: Thinly; thinness. Thin-skinned is attested from 1590s; the figurative sense of "touchy" is from 1670s.
thin (v.)
Old English þynnian "to make thin, lessen, dilute," also intransitive, "become thin," from thin (adj.). Intransitive sense of "to become less numerous" is attested from 1743; that of "to become thinner" is recorded from 1804. Compare similarly formed German dünnen, Dutch dunnen. Related: Thinned; thinning.
