yelp v.

Proto-Indo-European root meaning "to call." 

It forms all or part of: nightingale; yell; yelp. 

It is the hypothetical source of/evidence for its existence is provided by: Greek kikhle "thrush," also a kind of fish, khelidon "the swallow;" Lithuanian gulbinti "to praise;" Old English galan "to sing," galdor "spell, charm, magic, enchantment," giellan "to yell," gielpan "to boast."  

"fool, simpleton," 1825, perhaps from gawp (v.) "to yawn, gape" (as in astonishment), which is attested from 1680s, a dialectal survival of galp (c. 1300), which is related to yelp or gape and perhaps confused with or influenced by gawk.

c. 1300, yolpen, probably echoic variant of yelpen (see yelp). Related: Yawped; yawping. The noun, in reference to speech, is recorded from 1835, now used chiefly in conscious echo of Whitman (1855).