Foster Flop is an Australian saying, humorously describing the inability to get an erection after drinking too much beer.
It is comprised of "Foster", as in the famous Australian beer brand Foster's, and "Flop", as in "to fail completely" or "to fall heavily".
Last night after the party I wanted to root her but I Foster flopped.
Too bad you had a Foster Flop, otherwise we could have had sex.
"Did you have sex last night?" "No, I couldn't. I had a Foster Flop."