He: I can’t explain. It’s ineffable.
She: Are you saying I’m not f—able?
That’s right. Ineffable, describing something ‘that cannot be expressed or described in language’ or is ‘too great for words; transcending expression; unspeakable, unutterable, inexpressible’ is another one of those lonely negatives. Its positive form, effable, is used only as a snickery double entendre. (Or can it be a double meaning if one meaning is lost?)
“Effable” once meant ‘sounds or letters, etc. that can be pronounced.’ It came into English from the French effable, < Latin effābilis, < ef-fāri to utter, < ex out + fāri to speak. It is rarely used now, only as opposed to “ineffable” to mean ‘that which can be, or may lawfully be, expressed or described in words.’
Longfellow in The Divine Tragedy wrote of “These effable and ineffable impressions of the mysterious world.”