The "CM" part of CM Punk's name. It first stood for "Chick Magnet" (appropriately enough), but has long since lost its original meaning. It now means whatever the hell Punk wants it to mean at the time.
Triple H's Finishing Move, the Pedigree. It's a reference to his time as snotty BluebloodHeel Hunter Hearst Helmsley, a gimmick that he has long since abandoned. However, it is still occasionally referenced to whenever someone calls him "Hunter" instead of Triple H.
Chris Jericho debuted in WWE saying that he was the "Y2J Problem", playing off of the very 1999-specific reference to the Y2K bug, yet more than a decade and a half later, with the technological relevance of the reference long, long since passed, the Y2J nickname has stuck all these years.
The "Martial-Arts" part in Frontier Martial-Arts Wrestling was based in Atsushi Onita's original idea of creating a shoot-style promotion like Universal Wrestling Federation, in which he was negated the entry. This remained only as a name, as the company never featured shoot-style or martial arts and never had more than one acknowledge martial artist, Masashi Aoyagi.
Faarooq and Bradshaw's tag team, the "Acolyte Protection Agency", takes the "Acolyte" part from when they were members of The Undertaker's "Ministry of Darkness" stable. However after the end of that storyline they changed their gimmick to beer-drinking guns-for-hire with an "office" that only consisted of a free-standing door back stage and a card table, keeping the "Acolyte" part from their days as minions of a demonic overload (who subsequently became a biker...go figure).
In 1995, Jonathan Rechner debuted an evil Santa Claus gimmick called "Xanta Claus." His finisher was a Michinoku Driver variation called the Nutcracker Suite, a name that would stick long after he became Balls Mahoney. Then again, "nutcracker" can also refer to, well...