/səˈpoʊz/
[verb] To think that something is probably true, or to accept an idea as true for the purpose of an argument or discussion. It can also mean to expect or require something, as in 'you are supposed to be here at nine.'
Every time you say 'suppose,' you are literally placing an idea underneath reality to see if it holds the weight.
The word 'suppose' travels a remarkably straight road from Latin to modern English, and its meaning has stayed surprisingly close to its origins the whole way. It begins with the Latin verb 'ponere,' meaning 'to place' or 'to put.' This root is one of the great workhorses of Latin, and it generated dozens of compound verbs by combining with prepositions. In this case, the prefix 'sub-' meaning 'under' was attached to give 'subponere,' meaning 'to put under' or 'to place beneath.' This compound evolved and contracted in Vulgar Latin — the spoken, everyday form of Latin — into 'supponere,' which carried the sense of substituting one thing for another, or placing something underneath as a foundation or assumption.
From 'supponere,' the word passed into Old French as 'supposer,' where it began to take on the more intellectual flavour we recognise today: not just physically placing something beneath, but mentally placing an idea beneath an argument as its foundation. Old French 'supposer' meant to assume, to postulate, or to take something as a starting point for reasoning. This metaphor is quietly brilliant — an assumption is something you 'put under' your thinking to support it, the way a builder places a foundation beneath a house.
Middle English borrowed the word directly from Old French 'supposer' in the 14th century, writing it as 'supposen.' English speakers quickly took to it as a word for intellectual speculation and tentative belief. By the time Chaucer was writing in the late 1300s, 'suppose' was already part of the literary and scholarly vocabulary, used in exactly the way we use it now — to signal that something is assumed rather than proven.
Over the following centuries, the word's range of meaning quietly expanded. Beyond logical assumption, 'suppose' began to carry a softer, more social function — a polite way of agreeing reluctantly ('I suppose so'), or a gentle way of making a suggestion ('Suppose we try a different approach'). This social softening is common in English, where directness is often cushioned by words that frame an idea as merely hypothetical.
The passive construction 'supposed to,' meaning 'expected to' or 'required to,' developed somewhat later and represents one of the more interesting shifts in the word's life. When you say someone is 'supposed to' do something, you are drawing on the old idea of something being placed or established beneath a situation as its rule or expectation. This usage is now so common that many speakers do not connect it at all to the idea of supposing or assuming — yet the same Latin 'ponere' runs quietly through both. That same root also gave English 'position,' 'deposit' (Latin 'deponere' → 'deposer' in Old French → English 'deposit'), 'compose' (Latin 'componere' → Old French 'composer' → English 'compose'), and 'propose' (Latin 'proponere' → Old French 'proposer' → English 'propose') — each one a different way of placing something, somewhere, for some purpose.