ETYMOLOGY
from most (greatest in size, bulk, etc.) + what (pronoun); probably after somewhat (adv.)
EXAMPLE (for adv. 2.)
“… The parentes and freindes with whom I haue to deale, be mostwhat no latinistes: and if they were, yet we vnderstand that tounge best, whervnto we are first borne, as our first impression is alwaie in English, before we do deliuer it in Latin. …”
From: Positions vvherin those primitiue circumstances be examined, which are necessarie for the training vp of children, either for skill in their booke, or health in their bodie,
By Richard Mulcaster, 1581
Lexicophilia 