A threat. Used to indicate the speaker's intent to exact serious retribution from the owner of said ass. The literal threat is that the speaker
will take ownership of the ass as their property and
will freely penetrate the anus at their discretion, either for sexual pleasure, to inflict pain, or both. However, this literal act is rarely the intended form of punishment the speaker wishes to mete out. Usually, the speaker wishes to convey they
will exert the full extent of their authority over the other person to assert dominance over them. Loss of bodily autonomy illustrates submission.
Commonly used as an ultimatum; e.g., "if you
crash my car, your ass is mine."
Typically, this is used only amongst males where the threat of anal
rape is playful banter with a homoerotic undercurrent. If used against the opposite
sex, the threat of sexual violence carries a more severe
tone and would be highly inappropriate.
Related phrases:
You selling your ass,
boy?
I own your ass.