A greater degree than most. A level that cannot be beat. For example, to like the mostest would be the ultimate level of liking that cannot be surpassed.
I love my sweet lil' Ebeth (aka Baby) the mostest and that is fact.
Mostest, when someone calls mostest there is nothing that can beat Mostest so if they say tie they can not tie the person who called it wins. Only one person can call it the first to call it wins NO MATTER what.
Boy: love you
Girl: i love you mostest
Boy: no i do
After sometime....
Boy: lets call it a tie
Girl: there are no ties
Boy: dang it!