英语词源字典 - 在线词源网站

rare (adj.1)[rare 词源字典]: "unusual," late 14c., "thin, airy, porous;" mid-15c., "few in number and widely separated, sparsely distributed, seldom found;" from Old French rere "sparse" (14c.), from Latin rarus "thinly sown, having a loose texture; not thick; having intervals between, full of empty spaces," from PIE *ra-ro-, from root *ere- "to separate; adjoin" (cognates: Sanskrit rte "besides, except," viralah "distant, tight, rare;" Old Church Slavonic rediku "rare," Old Hittite arhaš "border," Lithuanian irti "to be dissolved"). "Few in number," hence, "unusual." Related: Rareness. In chemistry, rare earth is from 1818.[rare etymology, rare origin, 英语词源]
thin (adj.): Old English þynne "narrow, lean, scanty, not dense; fluid, tenuous; weak, poor," from Proto-Germanic *thunni "thin" (cognates: 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" (cognates: 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]
"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.
pho: "A type of Vietnamese soup, typically made from beef stock and spices to which noodles and thinly sliced beef or chicken are added", Vietnamese, perhaps from French feu (in pot-au-feu).