assumpsit (n.)

early 15c., "to arrogate, take upon oneself," from Latin assumere, adsumere "to take up, take to oneself, take besides, obtain in addition," from ad "to, toward, up to" (see ad-) + sumere "to take," from sub "under" (see sub-) + emere "to take" (from PIE root *em- "to take, distribute").

The meaning "to suppose, to take for granted without proof as the basis of argument" is recorded by 1590s; that of "to take or put on fictitiously" (an appearance, etc.) is from c. 1600. Related: Assumed; assuming.

The early past participle was assumpt. In rhetorical usage, assume expresses what the assumer postulates, often as a confessed hypothesis; presume expresses what the presumer really believes. Middle English also had assumpten "to receive up into heaven" (especially of the Virgin Mary), from the Latin past participle.