early 15c., originally in law, "action growing out of another action," from the adjective (see dependent) or from noun use of the adjective in French. It is attested from 1580s as "one who depends on or looks to another for support or favor."
As with its relative, dependence, it co-existed with the Latin-influenced variant (in this case dependent, from Latin dependere) through 18c., but with this word the French spelling (dpendant for both adjective and noun) has proven more durable in English, possibly because it has been found convenient to keep both, one (dependant) for the noun, the other (dependent) for the adjective.
But Century Dictionary (1897) places all senses under dependent, and writes:
As the spelling of this class of words depends solely upon whether they happen to be regarded as derived directly from the French or directly from the Latin, and as usage is divided, there is no good reason for insisting upon a distinction in spelling between the noun and the adjective ....