Apparently a middle voice of ephistemi (with nous implied); to put the mind upon, i.e. Comprehend, or be acquainted with -- know, understand.
see GREEK ephistemi
see GREEK nous
a. to be acquainted with: τί, Acts 18:25; James 4:14; Jude 1:10; τινα, Acts 19:15; with reference to what is said or is to be interpreted, to understand: Mark 14:68; 1 Timothy 6:4.
b. to know: περί τίνος, Acts 26:26; followed by an accusative with a participle Acts 24:10 (Winers Grammar, 346 (324); Buttmann, 301 (258)); followed by ὅτι, Acts 15:7; Acts 19:25; Acts 22:19; followed by ὡς, Acts 10:28; by πῶς, Acts 20:18; by ποῦ, Hebrews 11:8. (Synonym: see γινώσκω.) STRONGS NT 1987a: ἐπίστασιςἐπίστασις, ἐπιστασεως, ἡ (ἐφίστημι, ἐφισταμαι), an advancing, approach; incursion, onset, press: τῆς κακίας (Vulg.malorumincursio), 2 Macc. 6:3, where cf. Grimm; used of the pressure of a multitude asking help, counsel, etc., τίνι (on which dative cf. Winers Grammar, § 31, 3; (Buttmann, 180 (156)); Kühner, § 424, 1) to one, 2 Corinthians 11:28 L T Tr WH (but others would have us translate it here by oversight, attention, care, a common meaning of the word in Polybius); used of a tumultuous gathering in Acts 24:12 L T Tr WH. Cf. Buttmann, as above