From hupo and enantios; under (covertly) contrary to, i.e. Opposed or (as noun) an opponent -- adversary, against.
see GREEK hupo
see GREEK enantios
a. opposite to; set over against: ἵπποι ὑπεναντίοι ἀλληλοι, meeting one another, Hesiod scut. 347.
b. tropically (Plato, Aristotle, Plutarch, others), opposed to, contrary to: τινα, Colossians 2:14 (where see Lightfoot); ὁ ὑπεναντίος as a substantive (Xenophon, Polybius, Plutarch), an adversary, Hebrews 10:27, cf. the Sept. Isaiah 26:11 (the Sept. for אויֵב, צָר); often in the O. T. Apocrypha.