cover up for. Conceal a wrongdoing or wrongdoer, as in Bill was supposed to be on duty but went to a ballgame and Alan agreed to cover for him or I covered up for my friend when her mother called to find out where she was. [1960s] Also see cover up, def. 2.
Substitute for someone, act on someone's behalf, as in Mary was asked to cover for Joe while he was on jury duty. [c. 1970]
cover for something. Provide protection against some hazard, as in This policy covers the house for fire but not for theft. This idiom employs the verb to cover in the sense of “protect” or “shield,” a usage dating from the 13th century.