gymnasium n.

Old English nacod "nude, unclothed, bare; empty," also "not fully clothed" (a sense still used in 18c.),  from Proto-Germanic *nakwadaz (source also of Old Frisian nakad, Middle Dutch naket, Dutch naakt, Old High German nackot, German nackt, Old Norse nökkviðr, Old Swedish nakuþer, Gothic naqaþs "naked"), from PIE root *nogw- "naked" (source also of Sanskrit nagna, Hittite nekumant-, Old Persian *nagna-, Greek gymnos, Latin nudus, Lithuanian nuogas, Old Church Slavonic nagu-, Russian nagoi, Old Irish nocht, Welsh noeth "bare, naked").

Of things, "without the usual or customary covering" (of a sword, etc.), from Old English. Applied to qualities, actions, etc., "mere, pure, open to view, unconcealed," from c. 1200; phrase the naked truth is from early 15c. Phrase naked as a jaybird (1943) was earlier naked as a robin (1879, in a Shropshire context); Middle English had naked as a worm (mid-14c.), naked as a needle (late 14c.). Naked eye "the eye unassisted by any instrument" is from 1660s, an unnecessary term before telescopes and microscopes.

short for gymnasium, 1871, U.S. student slang.

1570s, "pertaining to athletic exercise," from Latin gymnasticus, from Greek gymnastikos "fond of or skilled in bodily exercise," from gymnazein "to exercise or train" (see gymnasium).