Asap - As soon as possible
Asap is something of an oddity amongst acronyms. Firstly, it is also an initialism, that is, it can be pronounced by spelling out the letters a. s. a. p. or as a word. Secondly, it is one of the few examples of an expression in the form 'as X as Y' that isn't a simile.
Like many acronyms it is of military origin. The first example that I can find of its use in print is from Captain Annis G. Thompson's account of the Korean War, The Greatest Airlift, 1954:
Emergency drops required no paperwork, merely a telephone call from the 8th Army in Korea. Most drops were made on an ASAP or 'as soon as possible' basis.
See other 'as x as y similes'.
See other phrases that were coined in the USA.