peculiar (adj.)

mid-15c., "belonging exclusively to one person," also "special, particular," from Old French peculiaire and directly from Latin peculiaris "of one's own (property)," from peculium "private property," literally "property in cattle" (in ancient times the most important form of property), from pecu "cattle, flock," related to pecus "cattle" (see pecuniary).

The meaning "unusual, uncommon, odd" is by c. 1600 (earlier "distinguished, special, particular, select," 1580s; for sense development, compare idiom). The euphemistic phrase peculiar institution for U.S. slavery is by 1838. Related: Peculiarly.