Member of the legal profession; someone who assists accident victims in getting compensation for avoidable or, more usually, unavoidable reasons (IE being compensated for tripping over a loose floor tile). Also used to describe anyone who profits from others' misfortune.
"That ambulance-chaser tried to get my aunt to sign a contract after her accident. She was still in her hospital bed!"
Pretty much self defining, one who chases around ambulances. Ok, to be more specific one who chases ambulances in order to file class action and personal injury lawsuits in the name of fighting for the little guy.