To greatly insult someone verbally (and sometimes physically) about themselves, situations they've been in, their family, friends, or anything under the sun. It is more of a competitive thing mostly used to see who can make the other person feel stupid. Types of jokes used are jokes about a person they look like, objects they look or smell like, a quote from a song, book, movie etc.. that they are relevant to, their clothing, etc. Most people use these to have a laugh, some people use it to piss the other person off (leading to fights).

There are certain words to use after every refernce such as

"Lookin ass"
"Body ass"
"Foot ass"
"Face ass"
"Back ass"
"Jaw ass"
"chin ass"
"havin ass"

or anything else having to do with the previous heat. Those are the more commonly used ones though.