Bacronyms are retrospective acronyms - a variety of folk etymology. Probably the best-known is the fictitious "Port Out, Starboard Home" for "posh".
Today, I managed to keep a friend in an exasperated superposition of belief/disbelief for about ten minutes with my spontaneously generated bacronym for "path" - "Public Access To Hill". I claimed this dated back to the 1860s, when a walking route in Glen Tilt, serving the mountain Beinn a' Ghlo, was so signposted, but subsequently closed off by the landowner, the Duke of Atholl. This (in my on-the-fly parallel universe) resulted in several mass trespasses, a campaign by the Scotsman newspaper ("Give Us Back Our P.A.T.H."), and an eventual revision to Scottish law enshrining the Right To Roam.
Any more?
Grant Hutchison