Origin: 1300–50; Middle English presumen (< Old French presumer) < Latin praesūmere to take beforehand (Late Latin: take for granted, assume, dare), equivalent to prae-pre- + sūmere to take up, suppose (see consume)
late 14c., "to take upon oneself, to take liberty," also "to take for granted, presuppose," from O.Fr. presumer (12c.), from L. præsumere (see presumption). Presumptive is recorded from 1560s. The heir presumptive (1620s) is "presumed" to be the heir if the heir apparent is unavailable.