Prodecessor of a long line of "Most" vocubaulary,
originally the phrase was "Doin' to much", which was said when someone was obsessing over something that simply shouldn't be obsessed about,
then it converted to Doing SO much that you'd end up "Doin' the Most", and to shorten it all up Many High Schools (Including IHS) came up with "Mostin it"
Boy: Man the french teacher be doin' to much, a test and a project as well as 2 pages of homework!!!
Girl: Yeah, He be Mostin It!