Achrēstos (ἄχρηστος) combines a (without) and chrēstos (useful, good). It means "useless, unprofitable." It appears once in Philemon 1:11, in Paul's wordplay on Onesimus's name (onesimos = "profitable"): formerly achrēstos (useless), now euchrēstos (useful).
Paul's wordplay in Philemon is one of the NT's most elegant — and theologically loaded — puns. Onesimus ("profitable") had been achrēstos (useless) as a runaway slave. After conversion, he became euchrēstos (genuinely useful). The gospel transforms people from what their names cannot claim into what only grace can accomplish. Paul's intercession for Onesimus prefigures Christ's intercession for us: just as Paul asked Philemon to receive the formerly useless slave as a beloved brother, Christ presents us to the Father — transformed, forgiven, and genuinely useful for the kingdom.