1660s, "bark as a (small) dog," earlier as a noun, "yapping dog" (c. 1600), probably of imitative origin. Compare verb yamph in same sense (1718). Originally in reference to dog sounds; meaning "to talk idle chatter" is first recorded 1886. Related: Yapped; yapping. As a noun, 1826 in reference to the sound; 1900, American English slang as "mouth."
-y 2
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy). Adjectives such as hugy, vasty are artificial words that exist for the sake of poetical metrics.