"Booty" I suspect is a variant pronunciation of "botty" (short for "bottom"), though there may also be an association with the alternative meaning of treasure.

As Amos says, the "Posh" story is nonsense. Widespread use of acronyms is a relatively recent phenomenon, & incorrect back formations of a number of words (such as "fuck") now exist.

Another recent occurence is back formation of words by adding or subtracting prefixes to reverse meaning. Usually this is incorrect usage. By way of example: Dis/gruntled - both words mean unhappy, irregardless doesn't suddenly become "full of consideration", nor does "dischuffed" become unhappy ("chuffed", in any case, has reversed it's meaning in the last 50 years, my dad remembered it as meaning angry, rather than pleased, when 'e were a lad...).

Often the supposed negative on the front of a word isn't any such thing. Instead, it's an intensive, so that "disgruntled" actually means "really gruntled" i.e. really pissed (off, that is! *g*). In this instance, to reverse the meaning you would probably have to say ungruntled or, more accurately, anti-gruntled. On the other hand, you could just stop being a pretentious pillock & simply use merry, happy, cheerful, gay, jolly, glad, or any one of many other perfectly good words to describe your positive mood!

(Not that I'm above using gruntled myself, but I normally use it in its correct context, not least because it confuses people! {Evil Grin})