I'm sure I've heard a verb that means this, but it might be colloquial, and possibly quite specific to a certain group of people.
The context is that person 'A' has tried to give a precise definition of some grand concept. For example, 'A' says "life can be defined as any system that maintains a low entropy by degrading an energy source" Person 'B' then gives a counterargument along the lines of "ah, but under that definition, wouldn't a candle flame also be considered alive?" In other words, 'B' has satisfied 'A's definition, but not in a way that 'A' will like, thus forcing 'A' to change her definition. 'B' can then be said to have ____ed 'A's original definition.
As Brian Donovan points out in a comment, the classic example is that when Plato defined 'human being' as 'featherless biped', Diogenes produced a plucked chicken, causing Plato to amend his definition. (To 'featherless biped with fingernails'.)
The word might also refer more generally to giving someone what they've asked for, but according to the letter rather than the spirit of what they said, thus not actually being what they wanted at all.
Invalidating a definition in this way is a special case of reductio ad absurdum of course, but I'm looking for a word that specifically describes this case.
I'm also not looking for general words meaning to invalidate or disprove; the word I'm thinking of (if it exists) refers very specifically to the case of showing that a definition is more broadly applicable than was originally intended.
The word I'm looking for (still, nearly three years later) might be somewhat humorous in nature.