late 14c., from Old French ineffable (14c.) or directly from Latin ineffabilis "unutterable," from in- "not, opposite of" (see in-(1)) + effabilis "speakable," from effari "utter," from ex- "out" (see ex-) + fari "speak" (see fame, n.). Plural noun ineffables was, for a time, a jocular euphemism for "trousers" (1823). Related: Ineffably.