"to set in, put or place in," 1520s, from Latin insertus, past participle of inserere "to graft, implant," from in- "in" (from PIE root *en "in") + serere "join together, arrange, put in a row," from PIE root *ser- (2) "to line up." Middle English had inseren "to set in place, to graft, to introduce (into the mind)" (late 14c.), directly from the Latin verb. Related: Inserted; inserting.
"something inserted," 1893, especially a paper, etc., placed in among the pages of a newspaper, magazine, etc., from insert (v.).