Old English mægen (n.) "power, bodily strength, force, efficacy," from Proto-Germanic *maginam "power," suffixed form of PIE root *magh- "to be able, have power." 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 nautical jargon used loosely for "the ocean," but in Spanish Main the word is short for mainland and refers to the coast between Panama and Orinoco (as contrasted to the islands of the West Indies).
early 13c., "large, bulky, strong," from Old English mægen- "power, strength, force," used in compounds (such as 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, African-American vernacular.