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 synonymous adjective suppositious is often preferred over supposititious.