Gerd Müller, perhaps uniquely among Germans, had his name appropriated as a piece of 70's British slang. Although admittedly a great soccer player and a prolific goal-scorer, Müller was not above hacking at the legs of opponents; in fact, the ferocity of his tackles was probably partly responsible for his incredibly goal-to-game ratio, since opposing players really did think twice before tangling with him.

In what was almost certainly, in part at least, grudging admiration, Müller's name became a part of the soccer vernacular: any player who had been injured in a particularly rough tackle was said to have been "mullered". Thus, both 'to muller' and 'to be mullered' became fairly standard expressions in English soccer, and subsequently in pretty much all forms of sport in the 70's. At its height, the expression could be used to refer to anything which had been damaged or worn out.

It was very much the slang of a particular era, and its use has since greatly declined. In fact, it's pretty much mullered.

Compare with 'knackered'.