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.
late 14c., "allied, united in a league," from Late Latin confoederatus "leagued together," past participle of confoederare "to unite by a league" (see confederate (v.)). Meaning "of or belonging to the Confederate States of America" is from 1861. Confederal "of or pertaining to a confederation" (1782) was used of the United States under the Articles of Confederation.
late 15c., "one who is united with another or others in a compact or league;" see confederate (adj.). Late Latin confoederatus also was used as a noun in its day. Meaning "a citizen of the Confederate States of America" is from 1861. Confederator (mid-15c.) was used in the sense of "an ally, an accomplice."