Entries linking to mainsail
early 13c., "notably large, bulky, or strong" (a sense now obsolete), from Old English mægen- "power, strength, force," used in compounds (such as mægensibb "great love," mægenbyrðen "heavy burden;" see main (n.)), probably also in part from or influenced by cognate Old Norse megenn (adj.) "strong, powerful, mighty."
Sense of "chief, principal, prime" is from c. 1400. That of "principal or chief in size or extent" is from 1590s. Main chance "opportunity of enriching oneself" is by 1570s, from the game of hazard. Main course in the meal sense attested from 1829. Main man "favorite male friend; hero" is by 1967, African-American vernacular.
"piece of shaped cloth spread so as to catch the wind and cause a vessel to move in water," Old English segl "sail, veil, curtain," from Proto-Germanic *seglom (source also of Old Saxon, Swedish segel, Old Norse segl, Old Frisian seil, Dutch zeil, Old High German segal, German Segel), of obscure origin with no known cognates outside Germanic (Irish seol, Welsh hwyl "sail" are Germanic loan-words). In some sources (Klein, OED) referred to PIE root *sek- "to cut," as if meaning "a cut piece of cloth."
As "a single ship or vessel" by 1510s. To take the wind out of (someone's) sails (1888) is to deprive (someone) of the means of progress, especially by sudden and unexpected action, "as by one vessel sailing between the wind and another vessel" ["The Encyclopaedic Dictionary," 1888].