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"believed or thought to exist," 1580s, past participle adjective from suppose (q.v.); often with the -e- pronounced, to distinguish it from the passive past tense supposed, now common in the sense of "to have a duty or obligation" (1859).
early 14c., "to assume as the basis of argument," from Old French supposer "to assume," probably a replacement of *suppondre (influenced by Old French poser "put, place"), from Latin supponere "put or place under," from sub "under" + ponere "put, place" (see position). Meaning "to admit as possible, to believe to be true" is from 1520s.