"put by artifice in place of another," 1610s, from Latin supposititius, from suppositus, past participle of supponere (see suppose).
Entries linking to supposititious
suppose v.
early 14c., "to assume as the basis of argument," from Old French suposer "to assume" (13c.), probably a replacement (influenced by Old French poser "put, place") of *suppondre, from Latin supponere "put or place under; to subordinate, make subject," from assimilated form of sub "under" (see sub-) + ponere "put, place" (past participle positus; see position (n.)). Meaning "to admit as possible, to believe to be true" is from 1520s.