early 15c., "act of confederating, alliance, agreement," from Anglo-French confederacion, Old French confederacion (14c.), from Late Latin confoederationem (nominative confoederatio), noun of action from past-participle stem of confoederare "to unite in a league," from assimilated form of com "with, together" (see con-) + foederare (from suffixed form of PIE root *bheidh- "to trust, confide, persuade").
Meaning "states or persons united by a league" is from 1620s. In U.S. history, the Articles of Confederation were adopted by Congress in 1777 and ratified by the states over the next four years. They went into effect March 1, 1781, and expired March 4, 1789.