Implacable - Vine's Expository Dictionary of New Testament Words
Implacable[ 1,,G786, aspondos ]
lit. denotes without a libation" (a, negative, sponde, "a libation"), i.e., "without a truce," as a libation accompanied the making of treaties and compacts; then, "one who cannot be persuaded to enter into a covenant," "implacable," 2 Timothy 3:3 (AV, "truce-breakers"). Some mss. have this word in Romans 1:31.
Note: Trench (Syn. lii) contrasts aspondos with asunthetos; See Note under COVENANTBREAKERS. Aspondos may signify "untrue to one's promise," asunthetos "not abiding by one's covenant, treacherous."