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 ....
On, Upon. These words are in many uses identical in force, but upon is by origin (up + on) and in use more distinctly expressive of motion to the object from above or from the side. On has the same force, but is so widely used in other ways, and so often expresses mere rest, that it is felt by careful writers to be inadequate to the uses for which upon is preferred. [Century Dictionary, 1895]
1590s, "condition of being logically dependent; relation of a thing or person to that by which it is supported;" 1610s, "that which depends for its existence upon something else;" see dependent + -cy. Originally also dependancy, on the French model, but the Latinate form gradually pushed this into disuse; see -ance and compare dependant (n.). Meaning "territory subordinate to another nation" is recorded from 1680s.
also dependant, late 14c., "relying for existence on;" early 15c. as "contingent, related to some condition;" from Old French dependant, present-participle adjective from dependre "to hang down," from Latin dependere "to hang from, hang down; be dependent on, be derived," from de "from, down" (see de-) + pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin").
For spelling, see dependant (n.). In some cases the English word is directly from Latin dependentem (nominative dependens), present participle of dependere. From early 15c. in the literal sense of "hanging down, pendent." From 1640s as "subordinate, under the control of or needing aid from an extraneous source." Dependent variable in mathematics is recorded from 1852.