prop (n.1)

"support," mid-15c., from Middle Dutch proppe "vine prop, support," of unknown origin. Probably related to Old High German pfropfo, German pfropfen "to prop," perhaps from Latin propago "a set, layer of a plant" (see propagation). Irish propa, Gaelic prop are from English.

prop (n.2)

"object used in a play," 1898, from props (1841), shortened form of properties (which was in theatrical use from early 15c.). Props as slang shortening for proper respects (or something similar) appeared c. 1999.

prop (v.)

"to support," mid-15c., probably from prop (n.1) or a related verb in Dutch. Related: Propped; propping.

prop (n.3)

short for propeller, 1914.

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Definitions of prop from WordNet
prop (n.)
a support placed beneath or against something to keep it from shaking or falling;
prop (n.)
any movable articles or objects used on the set of a play or movie;
before every scene he ran down his checklist of props
Synonyms: property
prop (n.)
a propeller that rotates to push against air;
Synonyms: airplane propeller / airscrew
prop (v.)
support by placing against something solid or rigid;
Synonyms: prop up / shore up / shore