English word presumably comes from English presume, English -ably

Detailed word origin of presumably

Dictionary entry	Language	Definition
presume	English (eng)	(intransitive) To be presumptuous; with on, upon, to take advantage (of), to take liberties (with). [from 15th c.]. (transitive) To assume to be true (without proof); to take for granted, to suppose. [from 14th c.]. (transitive) With infinitive object: to be so presumptuous as (to do something) without proper authority or permission. [from 14th c.]. (transitive, now, _, rare) To perform, do [...]
-ably	English (eng)	Used to form adverbs corresponding to adjectives that end in -able.
presumably	English (eng)	Able to be sensibly presumed.