protect one's ass

:  to cover or shield from exposure, injury, damage, or destruction :  guard

:  defend

:  to maintain the status or integrity of especially through financial or legal guarantees: such as

:  any of several hardy gregarious (see gregarious 1a) African or Asian perissodactyl mammals (genus Equus) smaller than the horse and having long ears

:  an African mammal (E. asinus) that is the ancestor of the donkey

:  a stupid, obstinate, or perverse person

:  buttocks

:  anus

: used as a postpositive intensive especially with words of derogatory implication