Convenient forgetfullness when it's beneficial not to remember the truth. Often seen during jury trials when a witness is asked an embarassing question.
I work my tail off all year but when it comes time for my performace review the boss always seems to have selective amnesia. Mr Oswald, what were you doing in that book depository? Uh, I dont' remember.
to benefit oneself by pretending to forget.
"Dude, if I loan you this twenty dollars please pay me back. Don't come down with a case of selective amnesia on me."