Etymology
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back off (v.)
"retreat, stop annoying someone," by 1938, from the verbal phrase, from back (v.) + off (adv.).
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post office (n.)

1650s, "public department in charge of letter-carrying," from post (n.3) + office. Meaning "building where postal business is carried on, office or place where letters are received for transmission," is from 1650s. In slang or euphemistic sense of "a sexual game" it refers to an actual parlor game first attested early 1850s in which pretend "letters" were paid for by kisses.

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bug off (v.)
"leave quickly," by 1956, perhaps from bugger off (see bugger (v.)), which chiefly is British (by 1920s) but was picked up in U.S. Air Force slang in the Korean War. Also see bug (v.3). To bug out "leave quickly, scram" is from 1953.
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beat off (v.)
"drive (something) away by violent blows," 1640s, from beat (v.) + off (adv.). Meaning "masturbate" is recorded by 1960s.
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set off (v.)

verbal phrase; see set (v.) + off (adv.). From 1590s as "make prominent by contrast," 1610s as "adorn." Intransitive sense of "start on or as on a journey" is from 1774. Meaning "separate from contect" (in typography) is from 1824; sense of "ignite, discharge, cause to explode" is from 1810.

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piss off (v.)

1958, intransitive, "go away," chiefly British; the transitive meaning "annoy (someone)" is by 1968, chiefly U.S.; from piss (v.) + off (adv.). Pissed off "angry, fed up" is attested by 1946 (Partridge says 1937); said to have been used in the military in World War II; in common use from 1970s.

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jerk off (v.)

slang, "perform male masturbation," by 1896, from jerk (v.) denoting rapid pulling motion + off (adv.). Compare come off "experience orgasm" (17c.). Farmer and Henley ("Slang and Its Analogues") also lists as synonyms jerk (one's) jelly and jerk (one's) juice. The noun jerk off or jerkoff as an emphatic form of jerk (n.2) is attested by 1968. As an adjective from 1957.

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ex officio 
Latin, "in discharge of one's duties," literally "out of duty," from ex "out of" (see ex-) + officio, ablative of officium "duty" (see office).
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get off (v.)
"escape," c. 1600, from get (v.) + off (adv.). Sexual sense attested by 1973.
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go off (v.)
1570s, of firearms, etc., "explode, be discharged;" see go (v.) + off (adv.); meaning "depart" is c. 1600; that of "deteriorate in condition" is from 1690s; that of "reprimand" is from 1941 (originally with at, since c. 2000 more often with on).
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