federal: [17] The modern political use of federal and its various derivatives is a comparatively recent development, ushered in by the formation of the USA in the late 18th century. Its original meaning was ‘of a league or treaty’ (it was formed from Latin foedus ‘league, treaty’, which came from the same ultimate Indo-European base – *bhidh-, *bhoidh- – as faith), and its application to a ‘joining together of states into a single unit’ seems to have arisen from such phrases as federal union, which would originally have meant ‘union by treaty’. => confide, defy, faith, perfidy
1640s, as a theological term (in reference to "covenants" between God and man), from French fédéral, an adjective formed from Latin foedus (genitive foederis) "covenant, league, treaty, alliance," from PIE *bhoid-es-, from root *bheidh- "to trust" (which also is the source of Latin fides "faith;" see faith).
Secular meaning "pertaining to a covenant or treaty" (1650s) led to political sense of "formed by agreement among independent states" (1707), from use of the word in federal union "union based on a treaty" (popularized during formation of U.S.A. 1776-1787) and like phrases. Also from this period in U.S. history comes the sense "favoring the central government" (1788) and the especial use of the word (as opposed to confederate) to mean a state in which the federal authority is independent of the component parts within its legitimate sphere of action. Used from 1861 in reference to the Northern forces in the American Civil War.
