Counter-strike terminology, derived from the fighter pilot term ace denoting a pilot who has shot down five enemy aircract.
Whereas an ace means to kill all 5 players on the other team in a single round, a mexican ace is achieved by getting 5 points with the help of bomb points. Since a successful bomb plant/defusal gives you 3 points but requires very little skill, it is referred to as a mexican ace.
KSharp: "holy shit i'm owning right now"
Bullseye: "that was a mexican ace you retarded defuse whore"