by 1914 (1903 as push), a word of uncertain origin, but there is no evidence for the common derivation from an acronym of port outward, starboard home, supposedly the shipboard accommodations of wealthy British traveling to India on the P & O Lines (to keep their cabins out of the sun); as per OED, see objections outlined in G. Chowdharay-Best in Mariner's Mirror, January 1971; also see here. The acronym story dates from 1955.
More likely it is from slang posh "a dandy" (1890), from thieves' slang meaning "money" (1830), originally "coin of small value, halfpenny," possibly from Romany posh "half" [Barnhart].
The cavalryman, far more than the infantryman, makes a point of wearing "posh" clothing on every possible occasion — "posh" being a term used to designate superior clothing, or articles of attire other than those issued by and strictly conforming to the regulations. [E. Charles Vivian, "The British Army From Within," London, 1914]
A separate word-thread derives from Old English swancor "pliant, bending," and from this comes swanky (n.) "active or clever young fellow" (c. 1500).
early 15c., "mythical, legendary," from Latin fabulosus "celebrated in fable;" also "rich in myths," from fabula "story, tale" (see fable (n.)). Meaning "pertaining to fable" is from 1550s. Sense of "incredible" first recorded c. 1600, hence "enormous, immense, amazing," which was trivialized by 1950s to "marvelous, terrific." Slang shortening fab first recorded 1957; popularized in reference to The Beatles, c. 1963.
Fabulous (often contracted to fab(s)) and fantastic are also in that long list of words which boys and girls use for a time to express high commendation and then get tired of, such as, to go no farther back than the present century, topping, spiffing, ripping, wizard, super, posh, smashing. [Gower's 1965 revision of Fowler's "Modern English Usage"]
Related: Fabulously; fabulousness.