1530s, "to unite in a league or alliance," from Late Latin confoederatus, past participle of confoederare "to unite by a league," from assimilated form of com "with, together" (see con-) + foederare, from foedus (genitive foederis) "a league," from suffixed form of PIE root *bheidh- "to trust, confide, persuade."
The older verb was confeder (late 14c.), from Old French confederer, Medieval Latin confederare. Related: Confederated; confederating.