Any open beer that has been sitting out at least overnight, and then still gets drank. usually by someone who wakes up still drunk from the night before, or a seasoned drinker after the beer runs out.
"where did you find that beer, I thot we were out?"
"we are, its a yesterbeer..."
"Whatta party last night, I really need something to wet my whistle! What is there to drink?"
" nothing left, just grab a yesterbeer!"