Definition
A warning to an individual who has greatly offended one's dignity or has acted or spoken insolently.
Origins
Ellipsis of "I'm not the one to mess with", "I'm not the one you think I am" or some comparable expression.
How People Actually Use It
Don't funk with me. You are biting off more than you can chew. I'm more than just talk. You are starting a fight with the wrong person. "I'm not the one"
""I'll beat your A$$" "I'm not the one""
Source: Urban Dictionary