The old expression finders keepers is based on the very dubious legal grounds that whoever finds something by chance is entitled to keep it. The phrase is sometimes extended as "losers weepers" meaning the person who did not make the find will just have to lament its loss.
The idea of the expression dates back to ancient times and a variant can be found in the work of the Roman dramatist Plautus.
Other songs featuring the expression include ones by:
"Emperor's New Clothes
" by Panic! at the Disco ("I'm taking back the crown. I'm all dressed up and naked. I see what's mine and take it. Finders keepers, losers weepers.")
"Break Your Heart
" by Taio Cruz (Whether or not you get it all together then it's finders keepers and losers weepers. See I'm not tryin' to lead you on, no I'm only tryin' to keep it real.)
" by Kelly Rowland ("Make you lose yourself, and finders keepers. It go green light, go Weezy go.")