Ask at the forum if you have an Ancient or Modern Greek query!
P. and V. ἄχρηστος, ἄχρειος, ἀνόνητος, ἀνωφελής, ἀσύμφορος, ἀνήνυτος (Dem. 209), P. ἀλυσιτελής, V. ἀνωφέλητος (also Xen.).
fruitless: P. ἄπρακτος, ἄκαρπος, V. ἀκάρπωτος.
vain: P. and V. κενός, μάταιος.