Etymologically, the word ἐχθρός has the same root of the Latin word extra: this emphasizes the original meaning of the lemma as indicating a person coming from outside, hostile because foreigner. In fact, to designate the enemy in war, the ancient Greek language uses the lemma πολέμιος, often as a substantive ("adversary"). As a result, it seems that in the classical Greek world the concept of being hostile is not semantically associated with the idea of the enmity in war, and ἐχθρός works simply as the opposite of φίλος. In the LXX, on the contrary, the lemma πολέμιος appears rarely. In fact, the technical meaning of "enemy of war", appropriate to πολέμιος, in the LXX is absorbed by ἐχθρός. Starting from these observations, the paper will explore the different meanings of the lemma ἐχθρός in the Greek world and in the LXX, where it occurs more than 400 times. It will also show the results of the corresponding entry written for the second volume of the Historical and Theological Lexicon of Septuaginta (ed. Eberhard Bons, Daniela Scialabba, with the scientific coordination of Anna Mambelli, Tübingen: Mohr-Siebeck, 2020-).