Putatively comes from putative + -ly; putative from Late Latin putativus ('supposed'), from Latin putare ('to think, reckon, consider'). The historical chain includes Latin putativus, Latin putare, and Proto-Indo-European *pewH-. The quoted older senses, "supposed" and "to think, reckon, consider", supply the semantic anchors for putatively. The formation is best read as suffixal and borrowed: the word putatively carries its older form-history into a compact English lexical shape. In present English it is used as a adv, "by general assumption; supposedly". Related English forms include putative, impute, compute, and dispute. The result is a adv whose etymology preserves both the older form and the later English specialization.