expression / godan ~る verb:

• to suddenly assume an antagonistic attitude; to suddenly become defiant; to turn aggressive (in one's manner)
• to moon; to point one's naked buttocks at someone; to give somebody the brown-eye

godan ~む verb / intransitive verb:

• to bite into (e.g. rope into skin); to cut into; to dig into (e.g. of fingernails)
• to eat into; to encroach; to make inroads; to penetrate (e.g. a market); to break into (e.g. first place); to erode; to cut into (e.g. time, savings)
• to be wedged (i.e. underwear pulled from the back, driving it between the buttocks) - vulgar