From ischus; to have (or exercise) force (literally or figuratively) -- be able, avail, can do(-not), could, be good, might, prevail, be of strength, be whole, + much work.
see GREEK ischus
1. to be strong in body, to be robust, to be in sound health: οἱ ἰσχύοντες, as a substantive, Matthew 9:12; Mark 2:17 (Sophocles Tr. 234; Xenophon, Cyril 6, 1, 24; joined with ὑγιαίνειν, id. mem. 2, 7, 7).
2. to have power (from Aeschylus down), i. e.
a. to have a power evinced in extraordinary deeds, i. e. to exert, wield, power: so of the gospel, Acts 19:20; Hebraistically, to have strength to overcome: οὐκ ἴσχυσαν (A. V. prevailed not i. e.) succumbed, were conquered (so יָכֹל לֹא, Genesis 32:26 (25)), Revelation 12:8; κατά τίνος, against one, i. e. to use one's strength against one, to treat him with violence, Acts 19:16.
b. equivalent to to be of force, avail (German gelten): Hebrews 9:17; τί, Galatians 5:6, and Rec. in Galatians 6:15.
c. to be serviceable: εἰς τί (A. V. good for), Matthew 5:13.
d. followed by an infinitive to be able, can: Matthew 8:28; Matthew 26:40; Mark 5:4; (Mark 9:18 (infinitive to be supplied)); Mark 14:37; Luke 6:48; Luke 8:43; (