federal (adj.) Look up federal at Dictionary.com
1640s, as a theological term, from French fédéral, from Latin foedus (genitive foederis) "covenant, league, treaty, alliance," 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.