supposititious

The Latin verb supponere, meaning "substitute," has several legitimate heirs in English, including "supposititious" (which dates from the early 17th century) and "supposition" (a 15th century addition). The "fraudulent" and "illegitimate" meanings of "supposititious" trace back to "supponere" in a fairly direct route, whereas the "imaginary" and "hypothetical" meanings were influenced by the meanings of "supposition." In legal contexts, "supposititious" is primarily used in its earlier senses, as in "a supposititious (fraudulent) will" or "the child was supposititious (illegitimate)." When something hypothetical is being considered, the adjective "suppositious" is often preferred over "supposititious."