To be willing and prepared to help or defend someone; to look out for someone in case they need assistance. Don't worry about those thugs, buddy. I'll watch your back if they ever come bother you again.You can always rely on your parents to watch your back.See also: back, watch
watch your (or someone's) back
protect yourself (or someone else) against danger from an unexpected quarter.See also: back, watchAcronymsSeeWYB