Etymology
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postulate (v.)

1530s, "nominate to a church office," from Medieval Latin postulatus, past participle of postulare "to ask, demand; claim; require," probably formed from past participle of Latin poscere "ask urgently, demand," from *posk-to-, Italic inchoative of PIE root *prek- "to ask questions." The meaning in logic, "lay down as something which has to be assumed although it cannot be proved" dates from 1640s, from a sense in Medieval Latin.

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postulate (n.)

1580s, "a request, demand, petition," from Latin postulātum "demand, request," properly "that which is requested," noun use of neuter past participle of postulare "to ask, demand; claim; require" (see postulate (v.)).

The sense in logic, "proposition proposed for acceptance without proof, something taken for granted," is from 1640s, from a sense in Medieval Latin. The meaning "self-evident practical proposition" is by 1751. The earlier noun in English was postulation "a petition, request" (c. 1400). Middle English also had postulate (adj.) "nominated to a bishopric or archbishopric" (mid-15c.).

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postulant (n.)

1759, "one who or that which demands or asks; candidate for membership in a religious order during the probationary period," from French postulant "applicant, candidate," literally "one who asks," from Latin postulantem (nominative postulans), present participle of postulare "to ask, demand" (see postulate (v.)).

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expostulation (n.)

1580s, "action of remonstrating in a friendly manner;" 1590s, "argumentative protest," from Latin expostulationem (nominative expostulatio) "a pressing demand, complaint," noun of action from past-participle stem of expostulare "demand urgently," from ex "from" (see ex-) + postulare "to demand" (see postulate (v.)).

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expostulate (v.)

1530s, "to demand, to claim," from Latin expostulatus, past participle of expostulare "to demand urgently, remonstrate, find fault, dispute, complain of, demand the reason (for someone's conduct)," from ex "from" (see ex-) + postulare "to demand" (see postulate (v.)). Friendlier sense of "to reason earnestly (with someone) against a course of action, etc." is first recorded in English 1570s. Related: Expostulated; expostulating.

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*prek- 

Proto-Indo-European root meaning "to ask, entreat." 

It forms all or part of: deprecate; deprecation; expostulate; imprecate; imprecation; postulate; pray; prayer; precarious; precatory; prithee.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit prasna-, Avestan frashna- "question;" Sanskrit prcchati, Avestan peresaiti "interrogates;" Latin precari "ask earnestly, beg, entreat;" Old Church Slavonic prositi, Lithuanian prašyti "to ask, beg;" Old High German frahen, German fragen, Old English fricgan "to ask" a question.  

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