Used from mid-17c. especially in law, and there via its appearance at the head of legal document involving seizure, deposition, etc. ("Certificate of caption"), the sense was extended to "the beginning of any document," and thence to "heading of a chapter or section of an article" (1789), and, especially in U.S., "description or title below an illustration" (1919).
Proto-Indo-European root meaning "to grasp."
It forms all or part of: accept; anticipate; anticipation; behave; behoof; behoove; cable; cacciatore; caitiff; capable; capacious; capacity; capias; capiche; capstan; caption; captious; captivate; captive; captor; capture; case (n.2) "receptacle;" catch; catchpoll; cater; chase (n.1) "a hunt;" chase (v.) "to run after, hunt;" chasse; chasseur; conceive; cop (v.) "to seize, catch;" copper (n.2) "policeman;" deceive; emancipate; except; forceps; gaffe; haft; have; hawk (n.); heave; heavy; heft; incapacity; inception; incipient; intercept; intussusception; manciple; municipal; occupy; participation; perceive; precept; prince; purchase; receive; recipe; recover; recuperate; sashay; susceptible.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit kapati "two handfuls;" Greek kaptein "to swallow, gulp down," kope "oar, handle;" Latin capax "able to hold much, broad," capistrum "halter," capere "to grasp, lay hold; be large enough for; comprehend;" Lettish kampiu "seize;" Old Irish cacht "servant-girl," literally "captive;" Welsh caeth "captive, slave;" Gothic haban "have, hold;" Old English hæft "handle," habban "to have, hold."
also Mr. Chad, simple graffiti drawing of a head peering over a fence or wall, with the caption, "Wot, no ______?" (the U.S. version usually had "Kilroy was here"), 1945, British, of unknown origin, a reaction to war-time shortages and rationing.
also doggie, "a little dog, a pet word for a dog," 1825, from dog (n.) + -y (3). Doggy-bag "bag provided by restaurants for customers to take home leftovers" (presumably to feed to the dog) is attested by 1962.
LIVING IT UP. Marveling at size of sirloin steaks, Dave and Betty celebrate $4-a-week raise at a restaurant dinner. They paid $3.50 each, left with enough uneaten steak in a "doggie bag" to feed themselves, not the dog, all next day. [Life magazine, photo caption from article on living economically, April 6, 1962]
As an adj. doggy is attested from late 14c., from -y (2). The word has been used in various formations at least since late 19c. to describe the rear-entry variant of the human sex act when one partner is on all fours.