Micca:

Actually, "gruntle" is an old word, but not what your friend was making it.

"disgruntle (esp. in pa disguntled)--whence disgruntlement (-ment)--combines dis-, apart,+ (now only dial) gruntle, to grunt often, to complain, a freq of GRUNT."   from ORIGINS, A Short Etymological Dictionary of Modern English.

Dave Oesterreich