"slice of bread and butter," 1855, northern English, from butter (n.) + -y (2).
Entries linking to butty
butter n.
Old English butere "butter, the fatty part of milk," obtained from cream by churning, general West Germanic (compare Old Frisian, Old High German butera, German Butter, Dutch boter), an early loan-word from Latin butyrum "butter" (source of Italian burro, Old French burre, French beurre), from Greek boutyron. This is apparently "cow-cheese," from bous "ox, cow" (from PIE root *gwou- "ox, bull, cow") + tyros "cheese" (from PIE root *teue- "to swell"); but this might be a folk etymology of a Scythian word.
The product was used from an early date in India, Iran and northern Europe, but not in ancient Greece and Rome. Herodotus described it (along with cannabis) among the oddities of the Scythians. In old chemistry, applied to certain substances of buttery consistency. Butter-knife, a small, dull knife used for cutting butter at the table, is attested from 1818.
-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.