Old English nacod "nude, bare; empty," also "not fully clothed," 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"). Related: Nakedly; nakedness. Applied to qualities, actions, etc., from late 14c. (first in "The Cloud of Unknowing"); phrase naked truth is from 1585, in Alexander Montgomerie's "The Cherry and the Slae":
Which thou must (though it grieve thee) grant
I trumped never a man.
But truely told the naked trueth,
To men that meld with mee,
For neither rigour, nor for rueth,
But onely loath to lie.
Phrase naked as a jaybird (1943) was earlier naked as a robin (1879, in a Shropshire context); the earliest known comparative based on it was naked as a needle (late 14c.). Naked eye is from 1660s, unnecessary in the world before telescopes and microscopes.