Etymology
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ajar (adv.)

"slightly open, neither open nor shut," 1718, also on a jar, on the jar, perhaps from Scottish dialectal a char "turned a little way," earlier on char (mid-15c.) "on the turn (of a door or gate)," from Middle English char "a turn," from Old English cier "a turn" (see chore). For first element see a- (1). For unusual change of ch- to j-, compare jowl.

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catch (n.)

late 14c., "device to hold a latch of a door," also "a trap;" also "a fishing vessel," from catch (v.). The meaning "action of catching" is attested from 1570s. The meaning "that which is caught or worth catching" (later especially of spouses) is from 1590s. The sense of "hidden cost, qualification, etc.; something by which the unwary may be entrapped" is slang first attested 1855 in writings of P.T. Barnum.

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knock (v.)

Old English cnocian (West Saxon cnucian), "to pound, beat; knock (on a door)," likely of imitative origin. Figurative meaning "deprecate, put down" is from 1892. Related: Knocked; knocking. Of engines from 1869. To knock back (a drink) "swallow quickly or at a gulp" is from 1931. Many phrases are in reference to the auctioneer's hammer, for example knock down (v.) "dispose of (something) at auction" (1760).

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rapper (n.)

"one who or that which raps" in any sense, 1610s; see rap (v.)). It could mean "door-knocker" (1630s), "spirit-rapper" (1755), "professional perjurer" (1840), prison slang for "prosecutor" in prison slang (1904), "itinerant antiques buyer," with a tinge of shadiness (1914). The hip-hop performance sense emerged c. 1979. Rapster is from 1772.

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lintel (n.)

"horizontal piece resting on the jambs of a door or window," early 14c., from Old French lintel "threshold" (13c., Modern French linteau), a word of uncertain origin, probably a variant of lintier, from Vulgar Latin *limitalis "threshold," or a similar unrecorded word, from Latin limitaris (adj.) "that is on the border," from limes (genitive limitis) "border, boundary" (see limit (n.)). Altered by influence of Latin limen "threshold."

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shut (v.)

Middle English shitten, sheten, "close (a door, window, gate, etc.); lock, fasten closed," from Old English scyttan "to put (a bolt) in place so as to fasten a door or gate, bolt, shut to; discharge, pay off," from West Germanic *skutjan (source also of Old Frisian schetta, Middle Dutch schutten "to shut, shut up, obstruct"), from PIE root *skeud- "to shoot, chase, throw." Related: Shutting.

The meaning "to close by folding or bringing together" is from mid-14c. That of "prevent ingress and egress" is from mid-14c. The sense of "to set (someone) free (from)," by c. 1500, is obsolete except in dialectal phrases such as get (or be) shut of (attested by 1570s). To shut (one's) mouth "desist from speaking" is recorded from mid-14c.

As a past-participle adjective, "made fast, closed, enclosed," by late 15c. As a noun, "action, time, or place of shutting," by 1660s.

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red-light (adj.)

a red light as a sign to stop is from 1849, long before traffic signals; see red (adj.1) + light (n.). As the name of a children's game (in reference to the traffic light meaning) it is recorded from 1953.

The use of red-light district to indicate a city district with many brothels is by 1896.

On a few blocks east of the Bowery, in what was known as the Red Light district, there are still a few houses of this character. The Red Light district was so called because the hall light in disreputable houses had a red globe or shone through red curtains covering the transom of the hall door. A red light before a cigar store, cider room or coffee room indicated its purpose. The Parisian licensed brothel has a red lantern with the number of the house over the door. [I.L. Nascher, M.D., "The Wretches of Povertyville," Chicago, 1909] 
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trap (n.)

"contrivance for catching unawares," late Old English træppe, treppe "snare, trap," from Proto-Germanic *trep- (source also of Middle Dutch trappe "trap, snare"), related to Germanic words for "stair, step, tread" (Middle Dutch, Middle Low German trappe, treppe, German Treppe "step, stair," English tread (v.)).

This is probably literally "that on or into which one steps," from PIE *dreb-, an extended form of a root *der- (1), base of words meaning "to run, walk, step." The English word is probably akin to Old French trape, Spanish trampa "trap, pit, snare," but the exact relationship is uncertain.

The sense of "deceitful practice, device or contrivance to betray one" is recorded from c. 1400. The meaning "U-shaped section of a drain pipe" is from 1833. Slang meaning "mouth" is from 1776. Speed trap is by 1908. Trap-door "door in a floor or ceiling" (often hidden and leading to a passageway or secret place) is attested from late 14c. (trappe-dore).

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clique (n.)

1711, "an exclusive party of persons; a small set, especially one associating to arrogate power or privilege," from obsolete French clique, which meant originally (14c.) "a sharp noise," also "latch, bolt of a door," from Old French cliquer "click, clatter, crackle, clink," 13c., echoic. Apparently this word was at one time treated in French as the equivalent of claque (q.v.) and partook of that word's theatrical sense.

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Star Chamber (n.)

late 14c., apartment in the royal palace at Westminster in which members of the king's council sat to exercise jurisdiction 14-15c., it evolved 15c. into a court of criminal jurisdiction, infamous under James I and Charles I for arbitrary and oppressive proceedings. Abolished 1641. Supposedly so called because gilt stars had been painted on the ceiling. Later there was a star on the door.

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