From a (as a negative particle) and a derivative of spendo; literally, without libation (which usually accompanied a treaty), i.e. (by implication) truceless -- implacable, truce-breaker.
see GREEK a
see GREEK spendo
1. without a treaty or covenant; of things not mutually agreed upon, e. g. abstinence from hostilities, Thucydides 1, 37, etc.
2. that cannot be persuaded to enter into a covenant, implacable (in this sense from Aeschylus down; especially in the phrase ἄσπονδος πόλεμος, Dem. pro cor., p. 314, 16; Polybius 1, 65, 6; (Philo de sacrif. § 4); Cicero, ad Att. 9, 10, 5; (cf. Trench, § lii.)): joined with ἄστοργος, Romans 1:31 Rec.; 2 Timothy 3:3.