The word is most probably a ghost-word and is not found in Page's edition of Stesichorus and Ibycus, two poets who allegedly used the word. If it ever existed, it is a privative of τερπνός meaning "unpleasant", but if such a word qualified sleep, it could well be reinterpreted as "sleepless" on the model of ἄγρυπνος which is given as equivalent by the Genuinum. Then an etymology (or a pun already in the original poem?) parsed it as "without sleep", and found its way down to Byzantine Etymologica. Choeroboscus, Epimerismi in Psalmos p. 77-78, gives the word as a regular privative compound of τερπνός (Τὰ εἰς ΟΣ ὀνόματα ἐν τῇ συνθέσει ἀναβιβάζουσι τὸν τόνον, κακὸς ἄκακος, σεμνὸς ἄσεμνος, τερπνὸς ἄτερπνος), which could confirm that the word was indeed used by some poets, with the expected meaning.