Presumably is how you say 'I think this is true, I have a good reason for thinking it, and nobody has confirmed it'. The lights are off, so presumably they have gone to bed. His car has gone — presumably he left early.
It marks a conclusion drawn from evidence, and it leaves room for you to be wrong. Put it at the front of the clause or straight after the subject: presumably she knew; she presumably knew. Presumably is not the same as supposedly, which hints that the speaker doubts what other people are claiming, and it sits above possibly and below certainly. The family around it is worth learning together: presume, to take something as true without proof; presumption; and presumptuous, which is a criticism — taking a liberty you have no right to take.
1adverbpresumablyUsed to say you believe something is true, judging from what you already know.
“The office was dark, so presumably everyone had gone home.”
synonyms: probably
Presumably is built in layers out of presume: presume, then presumable, 'able to be presumed', then presumably. The ending -ly is the one that turns an adjective into the way something is done or said, and here it packs a whole careful guess into a single word.
Proto-Indo-European
*h₁em-
Latin
praesūmere — to take beforehand, anticipate
Anglo-Norman
presumer
Middle English
presumen
English
presume
English, today
presumably
Built from
From presume + -ably, presumable + -ly. presume comes from Middle English presumen, from Anglo-Norman presumer and its source, Latin praesūmere (“to take beforehand, anticipate”), from prae- + sūmere (“to take”).
Etymology from Wiktionary, CC BY-SA