1645, as a theological term, from L.
foedus "covenant, league" (gen.
foederis), related to
fides "faith" (see
faith). Meaning "pertaining to a treaty" (1660) 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.
Federation is first attested 1721, from L.L.
foederationem, from L.
foederare "league together."
Federalism (1793) was coined by Burke.
Federalist "member or supporter of the Federal party in U.S. politics" is from 1787.
Fed slang for "officer of the FBI" is from 1916.