Etymology
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inquire (v.)
c. 1300, enqueren, anqueren, "to ask (a question), ask about, ask for (specific information); learn or find out by asking, seek information or knowledge; to conduct a legal or official investigation (into an alleged offense)," from Old French enquerre "ask, inquire about" (Modern French enquérir) and directly from Medieval Latin inquerere, from in- "into" (from PIE root *en "in") + Latin quaerere "ask, seek" (see query (v.)), in place of classical Latin inquirere "seek after, search for, examine, scrutinize." The English word was respelled 14c. on the Latin model, but half-Latinized enquire persists. Related: Inquired; inquiring.
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inquirer (n.)
1560s, "one who inquires, a seeker, an investigator," agent noun from inquire.
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inquiring (adj.)
"given to inquiry or investigation," 1590s, present-participle adjective from inquire (v.). Related: Inquiringly.
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enquire (v.)
alternative form of inquire, according to OED mainly used in sense of "to ask a question." Related: enquired; enquiring.
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inquiry (n.)
early 15c., enquery, "a judicial examination of facts to determine truth;" mid-15c. in general sense "attempt to learn something, act or fact of inquiring," probably an Anglo-French noun developed from enqueren "to inquire" (see inquire). Respelled from mid-16c. to conform to Latin.
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inquisitive (adj.)

late 14c., from Old French inquisitif, from Late Latin inquisitivus "making inquiry," from Latin inquisit-, past participle stem of inquirere "seek after, search for; examine, investigate" (see inquire).

An housbonde shal nat been Inquisityf of goddes pryuetee nor of his wyf. [Chaucer, "Miller's Prologue"]

Related: Inquisitively; inquisitiveness.

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inquisitor (n.)
c. 1400, "an inspector, one who makes inquiries," from Anglo-French inquisitour, Old French inquisiteur, or directly from Latin inquisitor "searcher, examiner; a legal investigator, collector of evidence," agent noun from Latin inquirere (see inquire). As the title of an officer of the Inquisition, from 1540s. Related: Inquisitorial. Of the fem. forms, inquisitress (1727) is senior to inquisitrix (1825).
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inquest (n.)
Origin and meaning of inquest
late 13c., enquest, an-queste "legal or judicial inquiry," especially one before a jury, from Old French enqueste "inquiry" (Modern French enquête), from Vulgar Latin *inquaestia (source also of Italian inchiesta), from Latin inquisita (res) "(a thing) looked into; an inquiry," from fem. past participle of Latin inquirere "to seek after, search for" (see inquire). The form with in- prevailed from 18c.
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inquisition (n.)
late 14c., "judicial investigation, act or process of inquiring," from Old French inquisicion "inquiry, investigation" (12c., Modern French inquisition), from Latin inquisitionem (nominative inquisitio) "a searching into, a seeking; legal examination, a seeking of grounds for accusation," noun of action from past participle stem of inquirere (see inquire).

In Church history, inquisitors were appointed from 382 C.E. to root out heretics; the ecclesiastical court appointed 13c. by Innocent III to suppress heresy never operated in Britain. The English word began to be used in this sense (and with a capital initial letter) after c. 1500, and usually refers to the office's reorganization 1478-1483 in Spain, where it fell under the control of the state as what is commonly called the Spanish Inquisition, noted especially for its severity, secrecy, and the number of its victims.
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in- (2)
Origin and meaning of in-

element meaning "into, in, on, upon" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "in," from PIE root *en "in."

In Old French (and hence in Middle English) this often became en-, which in English had a strong tendency to revert to Latin in-, but not always, which accounts for pairs such as enquire/inquire. There was a native form, which in West Saxon usually appeared as on- (as in Old English onliehtan "to enlighten"), and some of those verbs survived into Middle English (such as inwrite "to inscribe"), but all now seem to be extinct.

Not related to in- (1) "not," which also was a common prefix in Latin, causing confusion: to the Romans impressus could mean "pressed" or "unpressed;" inaudire meant "to hear," but inauditus meant "unheard of;" in Late Latin investigabilis could mean "that may be searched into" or "that cannot be searched into." Latin invocatus was "uncalled, uninvited," but invocare was "to call, appeal to."

The trouble has continued in English; the hesitation over what is meant by inflammable being a commonly cited example. Implume (1610s) meant "to feather," but implumed (c. 1600) meant "unfeathered." Impliable can mean "capable of being implied" (1865) or "inflexible" (1734). Impartible in 17c. could mean "incapable of being divided" or "capable of being imparted." Impassionate can be "free from passion" or it can mean "strongly stirred by passion." Inanimate (adj.) is "lifeless," but Donne uses inanimate (v.) to mean "infuse with life or vigor." Irruption is "a breaking in," but irruptible is "unbreakable."

In addition to improve "use to one's profit," Middle English also had a verb improve meaning "to disprove" (15c.). To inculpate is "to accuse," but inculpable means "not culpable, free from blame." Infestive has meant "troublesome, annoying" (1560s, from infest) and "not festive" (1620s). In Middle English inflexible could mean "incapable of being bent" or "capable of being swayed or moved." In 17c., informed could mean "current in information," formed, animated," or "unformed, formless" ("This was an awkward use" [OED]). Inhabited has meant "dwelt in" (1560s) and "uninhabited" (1610s); inhabitable likewise has been used on opposite senses, a confusion that goes back to Late Latin.

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