Old English strang "physically powerful, powerful in effect; forceful, severe, firm, bold, brave; constant, resolute; arduous, violent," from Proto-Germanic *strangaz (source also of Old Norse strangr "strong," Dutch streng "strict, rigorous," Old High German strang "strong, bold, hard," German streng "strict, rigorous"), possibly from PIE *strenk- "tight, narrow." Originally compared strenger, strengest (compare old/elder/eldest).
Grammatical sense, of noun and verb inflections, is first attested 1841, translating German stark, used in a grammatical sense by Jakob Grimm (the terms strong and weak better fit German inflections). Strong suit (1865) is from card-playing. Strong man "man of great strength" (especially one who displays it professionally) is recorded from 1690s; meaning "dominating man in a political organization" is from 1859.