From ischus; forcible (literally or figuratively) -- boisterous, mighty(-ier), powerful, strong(-er, man), valiant.
see GREEK ischus
a. of living beings: strong either in body or in mind, Matthew 12:29; Mark 3:27; Luke 11:21; Revelation 5:2; Revelation 10:1; Revelation 18:21; ἐν πολέμῳ, mighty i. e. valiant, Hebrews 11:34, cf. Revelation 19:18; of one who has strength of soul to sustain the assaults of Satan, 1 John 2:14; universally strong, and therefore exhibiting many excellences, 1 Corinthians 4:10 (opposed to ἀσθενής); comparitive, Matthew 3:11; Mark 1:7; Luke 3:16; mighty, — of God, 1 Corinthians 1:25; Revelation 18:8 (Deuteronomy 10:17; 2 Macc. 1:24, etc.); of Christ raised to the right hand of God, 1 Corinthians 10:22; of those who wield great influence among men by their rank, authority, riches, etc., τά ἰσχυρά equivalent to τούς ἰσχυρούς (on the neuter cf. Winer's Grammar, § 27, 5), 1 Corinthians 1:27 (οἱ ἰσχυροί τῆς γῆς, 2 Kings 24:15); joined with πλούσιοι, Revelation 6:15 (Rec. οἱ δυνατοί).
b. of inanimate things: strong equivalent to violent, ἄνεμος, Matthew 14:30 (T WH omit ἰσχυρόν); forcibly uttered, φωνή, Revelation 18:2 (Rec. μεγάλη) (Exodus 19:19); κραυγή, Hebrews 5:7; βρονταί, Revelation 19:6; λιμός, great, Luke 15:14; ἐπιστολαί (stern (forcible)), 2 Corinthians 10:10; strong equivalent to firm, sure, παράκλησις, Hebrews 6:18; fitted to withstand a forcible assault, πόλις, well fortified, Revelation 18:10 (τεῖχος, 1 Macc. 1:33; Xenophon, Cyril 7, 5, 7; πύργος, Judges 9:51). (Cf. δύναμις, at the end.)