as a modifier, attested by 1941. The expression is much older (1610s). Related: Do-it-yourselfer.
"to a considerable extent," expressing a degree less than very, 1560s, from pretty (adj.). Pretty much "in a considerable degree" is by 1660s.
Proto-Indo-European root meaning "strong, great." It forms all or part of: ameliorate; amelioration; meliorate; melioration; meliorism; multi-; multiform; multiple; multiply; multitude. It is the hypothetical source of/evidence for its existence is provided by: Greek mala "very, very much;" Latin multus "much, many," melior "better."