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Old English þynne "narrow, lean, scanty," from Proto-Germanic *thunnuz, *thunw- (cf. West Frisian ten, Middle Low German dunne, Dutch dun, Old High German dunni, German dünn, Old Norse þunnr), from PIE *tnus-, *tnwi-, from weak grade of root *ten- "stretch" (cf. Latin tenuis "thin, slender;" see tenet).
These our actors ... were all Spirits, and Are melted into Ayre, into thin Ayre. [Shakespeare, "The Tempest," IV.i.150, 1610]Thin-skinned is attested from 1590s; the figurative sense of "touchy" is from 1670s.
Old English þynnian "to make thin" (cf. German dünnen, Dutch dunnen), from thin (adj.). Intransitive sense of "to become less numerous" is attested from 1743; that of "to become thinner" is recorded from 1804. Related: Thinned; thinning.