Definition of crunches:
(v) :
To crush something, especially food, with a noisy crackling sound.
(v) :
To be crushed with a noisy crackling sound.
(n) :
A noisy crackling sound; the sound usually associated with crunching.
(v) :
(slang) To calculate or otherwise process (e.g. to crunch numbers: to perform mathematical calculations). Presumably from the sound made by mechanical calculators.
(v) :
To emit a grinding or crunching noise.
(v) :
To grind or press with violence and noise.