To be willing and prepared to help or defend someone; to look out for someone in case he or she needs assistance. Don't worry about those thugs, buddy. I've got your back if they ever come bother you again. You can always rely on your parents to have your back. Bullfinch)
help out(to help sb, especially in a difficult situation Х He's always willing to help out. Х When I bought the house, my sister helped me out with a loan. OALD Alexander Demidov)
come to assistance(come to someone's assistance to arrive and provide assistance to someone. A kindly truck driver came to our assistance, and we were able to call for help. I hope someone will come to my assistance soon. See also: come McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. й 2002 Alexander Demidov)
