IPA: //ˈmɛs.meɪt//
KK: /ˈmɛsˌmeɪt/
A person who regularly shares meals with others, especially in a military or naval setting.
During the deployment, he became good friends with his messmate.
Messmate → The word originates from the combination of 'mess' (from Old French *mes*, meaning a meal or dish) and 'mate' (from Old English *gemetta*, meaning companion or associate). A messmate is a companion with whom one shares a meal.
Think of sharing a 'meal' ('mess') with a 'companion' ('mate') — that's what a messmate is.