What is the actual spelling/pronunciation? What is the origination of this phrase?
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Supposed to is a very common phrase, as in:
To suppose something means to assume something. When used in a passive construction, you use a form of the verb to be plus the past participle of to suppose, which is supposed. To be supposed to means to be expected to, to be obliged to. You can see that the verb gets a somewhat different sense in the passive voice from what would normally be expected.
The original meaning of "suppose" is epistemic: "think", or "assume".
But various modal verbs, such as "must", have both deontic ("you must go" = "you ought to go") and epistemic ("you must be" = "I conclude that you are") meanings.
In the passive only, "suppose" has acquired a deontic sense.
Nowadays "suppose" nearly always takes a sentential complement ("I suppose that ... "), but in older or more formal English it can take a direct object and an deonticinfinitive complement "I supposed him to be ... ".
So "He was supposed to go" is the passive of a construction which is now rather restricted, in the epistemic sense of "It was thought that he was going". But in the specialised deontic sense, the construction has survived.