*As it's widely believed

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"as may be supposed," 1795, not originally American English, alteration of supposedly , or else from supposable (1680s), from suppose (v.) + -able .

Wiktionary Word definitions in Wiktionary
adv. 1 able to be supposed or conjectured. 2 (context nonstandard English) supposedly.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Supposable \Sup*pos"a*ble\, a. Capable of being supposed, or imagined to exist; as, that is not a supposable case. -- Sup*pos"a*ble*ness , n. -- Sup*pos"a*bly , adv.