The article examines the meaning and word-formation of the hapax legomenon ἀχρηστομάθεια, which describes activities of the early Greek philosophers in Eusebius’ Praeparatio Evangelica. On the surface it may seem that α privativum refers to the first element of the compound and the lexeme itself has the meaning ‘knowledge of useless things’. However, we suppose that the negation pertains not to the first but more likely to the second part of the compound (-μάθεια). In this case ἀχρηστομάθεια would not imply that Greek philosophers pursue some unnecessary things but indicate the fact that they take absolutely no interest in important and useful matters (χρηστά).