Origin and history of suppositional

early 15c., supposicioun, a term in Scholastic logic, "admission of a likelihood based on the truth of its consequences, assumption for the sake of argument, hypothetical inference, conjecture," from Medieval Latin suppositionem (nominative suppositio) "assumption, hypothesis, a supposition," noun of action from past-participle stem of supponere (see suppose).

In classical Latin, "a putting under, substitution;" it translated Greek hypothesis. (see hypothesis), which influenced the sense. In English as "act of formulating a proposition, without regard to truth or falsehood," by 1590s. Often in general use in English vaguely, "notion, idea, fancy." Earlier in English the logical sense was in supposal (late 14c.). Related: Suppositionally.

suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).