ineffable (adj.)

late 14c., "beyond expression, too great for words, inexpressible," 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 assimilated form of ex "out" (see ex-) + fari "to say, speak," from PIE root *bha- (2) "to speak, tell, say."

Meaning "that may not be spoken" is from 1590s. Plural noun ineffables was, for a time, a jocular euphemism for "trousers" (1823; see inexpressible). Related: Ineffably.