"in two ways," a modern word-forming element extracted late 16c. from parti-colored (q.v.). In that word it represents Middle English partie "of two different colors; different," from Old French partie.
Entries linking to parti-
parti-colored adj.
1530s, party-colored, "colored differently in different parts," from party "divided," from Middle English partie "of different colors; different" (late 14c.), from French parti, past participle of partir "to divide," from Latin partiri "to share, part, distribute, divide," from pars "a part, piece, a share" (from PIE root *pere- (2) "to grant, allot"). The noun parti itself occurs in the sense "parti-colored" from late 14c. Also parti-coloured.