"theft; wrongful or fraudulent taking of the personal goods of another with felonious intent," late 15c., from Anglo-French larcin (late 13c.), Old French larrecin, larcin "theft, robbery" (11c.), from Latin latrocinium "robbery, freebooting, highway-robbery, piracy," from latro "robber, bandit," also "hireling, mercenary," ultimately from a Greek source akin to latron "pay, hire, wages," from a suffixed form of PIE root *le- (1) "to get" (source also of Greek latreia "worship, service paid to the gods, hired labor," latron "pay, hire," latris "servant, worshipper").
Perhaps with -y (3) added in English or else the word was altered by influence of burglary, felony. Formerly distinguished into grand larceny, involving property valued in excess of a stated amount, and petty larceny.
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."