Other forms: supposed, supposes, supposing
Suppose means you think something is true without being sure enough to swear to it. Someone asks whether the shop opens on Sunday and you say I suppose so — half an answer, half a shrug.
Suppose also sets up an imaginary situation: suppose it rains on the day, what then? Maths and arguments use it the same way — suppose x is 5. The past form hides a trap, because be supposed to means expected or required, not thought likely: I'm supposed to be at work at eight is about duty, not guessing. Supposedly puts distance between you and a claim you doubt, as in he was supposedly in Paris that week. And a supposition is the noun for the guess itself.
1verbsupposeTo think something is probably true without having proof.
“I suppose she missed the last train home.”
2verbtakeTo take something as true in order to see what follows from it.
“Suppose the power fails in the middle of the exam — what happens to your answers?”
3verbexpectedIn be supposed to: to be expected or required to do something.
“You were supposed to lock the back door before you left.”
Too few real examples of this word to estimate which sense is commonest.
Also accepted in games: assume
Suppose came through Middle English supposen from Old French supposer, built from sub-, 'under', and poser, 'to place' — to put an idea underneath. It answers to Latin supponere, 'to put under, to substitute, falsify, counterfeit'. A supposition is the base you slide under an argument to see whether the argument stands on it.
Proto-Indo-European
*tḱey-
Latin
supponere — to put under, to substitute, falsify, counterfeit
Old French
supposer
Middle English
supposen
English, today
suppose
Built from
From Middle English supposen, borrowed from Old French supposer, equivalent to prefix sub- (“under”) + poser (“to place”); corresponding in meaning to Latin supponere (“to put under, to substitute, falsify, counterfeit”), suppositum. See pose.
Etymology from Wiktionary, CC BY-SA