Confederate descends from Latin confoederātus (united by treaty), from con- (together) + foedus (treaty), related to fidēs (faith), PIE *bʰeydʰ- (to trust). A confederacy is a joining together in faith. The word's most famous use — the Confederate States of America — shares its root with its opponent's name, Federal, creating the irony that both sides of the Civil War were named for bonds of trust.
United in a league or alliance; joined by a treaty or compact. As a noun: a person or state joined in an alliance; an accomplice. As a verb: to unite in a league.
From Latin 'confoederātus,' past participle of 'confoederāre' (to unite by a league or treaty), a compound of 'con-' (together, with, from PIE *kom) + 'foederāre' (to establish by treaty), from 'foedus' (treaty, league, covenant, compact), genitive 'foederis.' 'Foedus' is related to Latin 'fidēs' (faith, trust, loyalty) and 'fīdus' (faithful, trustworthy), all from PIE *bʰeydʰ- (to trust, to persuade). The semantic core is trust expressed
Both 'Federal' and 'Confederate' derive from the very same Latin word — foedus, meaning 'treaty.' The American Civil War was, etymologically, a war between two sides whose names both meant 'bound together by agreement.' Switzerland captured this meaning literally in its official Latin name, Confoederatio Helvetica — hence the country