A dagger carried in the left hand to parry a stroke or thrust, and used in conjunction with a rapier.
main gauche/ˈmaŋ ˌɡəʊʃ/ /ˈmã ˌɡəʊʃ/
Mid 19th century; earliest use found in Charles Boutell (1812–1877), writer on heraldry and antiquities. From French main gauche, lit. ‘left hand’ from main hand + gauche left.