"bug"="defect"

     [This is a fast-access FAQ excerpt.]
The 1947 incident often related by Grace Hopper, in which a
technician solved a glitch in the Harvard Mark II computer by
pulling a moth out from between the contacts of one of its relays,
*did* happen.  However, the log entry ("first actual case of bug
being found") indicates that this is *not* the *origin* of this
sense of "bug".  It was used in 1899 in a reference to Thomas
Edison.  See the Jargon File.  It may come from "bug" in the
obsolete sense "frightful object", whose use in Coverdale's 1535
translation of the Bible led to its being nicknamed "the Bug Bible".
(Coverdale rendered Psalm 91:5 as "Thou shalt not nede to be
afrayed for eny bugges by night"; the King James Version reads
"terror".)  This word, which was the source of the current word
"bugbear" and may be related to "bogey", comes from Middle English
"bugge", which was used in the senses "scarecrow" and "demon".
Possible etyma are Welsh bwg="ghost" and proto-Germanic *bugja=
"swollen up, thick".  The latter is also posited as the etymon of
the Norwegian dialect bugge="important man" and English "big",
from the proto-Indo-European *beu-="to blow up, swell", whence the
English words "poach"="cook", "pocket", "poke"="bag", "pout",
"Puck"="sprite", and "pucker".
   "Bug"="insect" (which gave rise to the senses "germ", "annoy",
"enthusiast", and "listening device") is attested from 1622.  It may
come from Anglo-Saxon budda="beetle", influenced by "bug"=
"frightful object".