Entry 1 sense Webster, 1913 Mainsail /(mān"sāl`)/ · Main·sail · IPA /ˈmeɪnseɪl/ 01 n. The principal sail in a ship or other vessel. 1. The principal sail in a ship or other vessel.(Naut.) “[They] hoised up the mainsail to the wind.” — Acts xxvii. 40.