Why is the ninth month called September?
late 14c., "advantageous, fit, proper," from Old French expedient (14c.) or directly from Latin expedientem (nominative expediens) "beneficial," present participle of expedire "make fit or ready, prepare" (see expedite).
The noun meaning "a device adopted in an exigency, a resource" is from 1650s. Related: Expediential (1836); expedientially (1873); expediently (late 14c.).