μὴ κακὸν εὖ ἔρξῃς· σπείρειν ἴσον ἔστ' ἐνὶ πόντῳ → do no good to a bad man; it is like sowing in the sea
Ar. and P. ἐπίπονος, χαλεπός, P. and V. βαρύς, προσάντης (Plato), V. πολύπονος, δυσπόνητος; see laborious.