presumptuous adj.

late 14c., presumen, "to take upon oneself, to take liberty," also "to take for granted, believe or accept upon probable evidence, presuppose," especially overconfidently, from Old French presumer (12c.) and directly from Latin praesumere "anticipate," in Late Latin, "assume," from prae "before" (see pre-) + sumere "to take, obtain, buy," from sus‑, variant of sub‑ "up from under" + emere "to take" (from PIE root *em- "to take, distribute").

To presume is to base a tentative or provisional opinion on such knowledge as one has, to be held until it is modified or overthrown by further information. [Century Dictionary]

The intransitive sense of "to venture beyond the limits of ordinary license or propriety" and that of "to press forward presumptuously" are from early 15c. Related: Presumed; presumedly; presuming; presumingly.

mid-13c., presumpcioun, "seizure and occupation without right," also "taking upon oneself more than good sense and propriety warrant," from Old French presumcion (12c., Modern French présomption) and directly from Late Latin praesumptionem (nominative praesumptio) "confidence, audacity," in classical Latin, "a taking for granted, anticipation," noun of action from past-participle stem of praesumere "to take beforehand," from prae "before" (see pre-) + sumere "to take, obtain, buy," from sus‑, variant of sub‑ "up from under" + emere "to take" (from PIE root *em- "to take, distribute").

In English, the meaning "the taking of something for granted" is attested from c. 1300; that of "a ground for presuming or believing" is from 1580s. Presumptuous preserves the older sense.