This phrase is used to denote the uselessness of the word "if" after the fact. The entire phrase is usually given as "if a frog had wings, it wouldn't bump its ass when it hopped".
From the movie Wayne's World:
Cassandra: Yeah, and if a frog had wings, it wouldn't bump its ass when it hopped.
Wayne: Where exactly did you learn english?
Cassandra: College. And the Police Academy movies.
A modified form appears in the first season of the Drew Carey Show:
Drew: "hey, if a frog had wings... we'd still be screwed, but wouldn't it be cool?"
That definition isn't right. The phrase "If a frog had wings, he wouldn't bump his ass a-hoppin'" means that if a person didn't have to put up with a lot of shit or deal with a shitty situation, they wouldn't. If the frog had wings, he wouldn't bump his ass a-hoppin'.
Jim: Hey Bob, how come every girl you have sex with is a real brown bagger
Bob: If a frog had wings...