"cuckoo," early 14c., from Old Norse gaukr, from Proto-Germanic *gaukoz (source also of Old English geac "cuckoo," Old High German gouh). Meaning "fool" attested from c. 1600.
Entries linking to gowk
cuckoo n.
European bird noted for its love-note cry and notorious for parasitism, c. 1300, cokkou (late 12c. as a surname), from Old French cocu "cuckoo," also "cuckold," echoic of the male bird's mating cry (compare Greek kokkyx, Latin cuculus, Middle Irish cuach, Sanskrit kokilas).
Slang adjectival sense of "crazy" is American English, 1918, but noun meaning "stupid person" is recorded by 1580s, perhaps from the bird's unvarying, oft-repeated call. The Old English name was ʒeac, cognate with Old Norse gaukr, source of Scottish and northern English gowk, which also has insulting senses. The Germanic words presumably originally were echoic, too, but had drifted in form. Cuckoo-clock is from 1789.
gawk v.
"stare stupidly," 1785, American English, of uncertain origin. Perhaps [Watkins] from gaw, a survival from Middle English gowen "to stare" (c. 1200), from a Scandinavian source akin to Old Norse ga "to heed," from Proto-Germanic *gawon, from PIE *ghow-e- "to honor, revere, worship" (see favor (n.)); and altered perhaps by gawk hand (see gawky). Liberman finds this untenable and writes that its history is entangled with that of gowk "cuckoo," which is from Scandinavian, but it need not be from that word, either. Nor is French gauche (itself probably from Germanic) considered a likely source. "It is possibly another independent imitative formation with the structure g-k" (compare geek). From 1867 as a noun. Related: Gawked; gawking.