1640s, as a theological term, from L. foedus
"covenant, league" (gen. foederis
), related to fides
"faith" (see faith
). Meaning "pertaining to a treaty" (1650s) led to political sense of "state formed by agreement among independent states" (1707), from phrases like federal union
"union based on a treaty," popularized by formation of U.S.A. 1776-1787.