before 900; (noun) Middle Englishmeyn, mayn strength, power, Old Englishmægen, cognate with Old Norsemegin(n), megn strength; (adj.) Middle Englishmayn, partly < Old Norsemegenn, megn strong, partly independent use of Old Englishmægen (noun) taken as an adj. in compounds, as in mægen-weorc, literally, work of might
Old English mægen (n.) "power, bodily strength, force, efficacy," from Proto-Germanic *maginam "power," suffixed form of PIE root *magh- (1) "be able, have power" (see may (v.)). Original sense preserved in phrase with might and main. Meaning "principal channel in a utility system" is first recorded 1727 in main drain; Used since 1540s for "continuous stretch of land or water." In Spanish Main the word is short for mainland and refers to the coast between Panama and Orinoco.
early 13c., "large, bulky, strong," from Old English mægen- "power, strength, force," used in compounds (e.g. mægensibb "great love," mægenbyrðen "heavy burden;" see main (n.)), probably also from or influenced by Old Norse megenn (adj.) "strong, powerful." Sense of "chief" is c.1400. Main course in the meal sense attested from 1829. Main man "favorite male friend; hero" is from 1967, U.S. black slang.