ABC is Accidental Breast Contact. That is, one or more breasts coming into contact with usually arm or back in a social situation. It should perhaps be divided into two subsets, AABC, Apparently Accidental Breast Contact, and possibly SDAABC, Subconsciously Directed Apparently Accidental Breast Contact. ABC is a step beyond flirting. ABC means 'I want to sleep with you'.
The ABC thesis is that a woman will never touch a male of breeding age with her breasts by accident. Women are usually quite careful what they do with their secondary sexual characterisics around men, living as they do in a society that reinforces the idea that cunning use of one's physical attributes is the best strategy for the young female in almost any given situation.
In personal experience, women deny that my theory has any basis in reality. This leads me to believe that either I am deluded and a borderline misogynist, or that people are often less aware of the true motivations for our actions than we would like to think.