Etymology
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walk-over (n.)
"easy victory," 1838, such as one that happens in the absence of competitors, when the solitary starter, being obliged to complete the event, can traverse the course at a walk. Transferred sense of "anything accomplished with great ease" is attested from 1902. To walk (all) over (someone) "treat with contempt" is from 1851.
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walk-out (n.)
"strike," 1888, from walk (v.) + out (adv.). Phrase walk out "to leave" is attested by 1840. To walk out on a person "desert, forsake" is by 1913.
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clip-on (adj.)

"held on by means of a clip," 1909, from the verbal phrase; see clip (v.2) + on (adv.).

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walk-in (adj.)
1928, "without appointment," from the verbal phrase, from walk (v.) + in (adv.). As a noun, meaning "walk-in closet," by 1946.
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walk-up (adj.)
in reference to an apartment not accessible by elevator, 1909, from the verbal phrase; see walk (v.) + up (adv.). As a noun from 1920 in reference to that type of apartment.
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hard-on (n.)
"penile erection," 1922, earlier as an adjective (1893), from hard + on.
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add-on (n.)
"additional component," 1941, from verbal phrase add on; see add (v.) + on (adv.).
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air-rifle (n.)
rifle that uses compressed air power to fire the projectile, 1851, from air (n.1) + rifle (n.).
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air-space (n.)

also airspace, by 1821 in reference to stove and furnace construction, from air (n.1) + space (n.). From 1852 in reference to the cubic contents of a room (with reference to the persons in it) in sanitary regulations for boarding rooms, hospitals, etc. In firearms, "a vacant space between the powder charge and the projectile" (1847). By 1910 as "portion of the atmosphere controlled by a country above its territory."

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