I thought the “Prisoner” acronym had been debunked as a myth a long time ago, as suggested by Giles’s snopes link.
Oh shit. This is going to sound like major backpedalling, but I meant to say I believeved the “immigrant” one rathenr than the P.O.H.M. one. Yeesh. Brainfart.
So you’re not calling them pom-pom girls (cheerleaders?) 
That would have been a girly-man sort of insult. 
The word for the frilly things waved by cheerleaders is actually pompon, from the French. Changing the last letter to -m is an example of linguistic assimilation, and in this case it’s the fault of us hopeless Americans, who love to goof up the French language every chance we get. (And don’t even get me started on “chaise lounge” instead of chaise longue. Sheesh!)
Yes, there’s a whole catalog of folklore about us Yanks butchering French (and of course, English) words. I even heard of a newly-converted Yank Jew who blew his chauffeur to mark a holy day. 
;j
Any etymology prior to WW2 involving acronyms - with the exception of OK and possibly AWOL - is almost certainly apocryphal: the only exception I’ve been able to reliably verify is the aircraft manufacturer Avro, an abbreviation of A. V. Roe & Company Ltd., which dates back to 1913:link and link.
There’s an earlier thread on the subject here, which also deals with the Greek acronym ichthys. Cecil touches on whether or not the “hip” in “Hip Hip Hooray” was originally an anti-Semitic acronym here, but I have to say I’m less than convinced, given the murky etymology of “hooray” which he neglects entirely.