Origin: Latin prefix pre-
Presumably has 2 different meanings across 1 category:
able to be sensibly presumed
"The package was delivered presumably because the tracking system showed it had arrived at our doorstep."
In plain English: Presumably means you think something is probably true because there are good reasons to believe it, even though you haven't checked for sure yet.
"She will be there presumably by five o'clock if traffic is light."
Presumably comes from combining the word presume with the suffix -ably to form presumable, and then adding -ly. This construction created an adverb meaning "as is presumed" by extending the base verb into a descriptive modifier.