Chalker is a fat person, specifically a girl. A girl is a Chalker when they are too big for you to wrap your arms around them when you hug them. In this situation you put pieces of chalk in your hands and mark down on the girl where your hands end. Then you would continue to the other side of the fat girl and put your hands where the chalk lines are, completing the hug.
Basically: A chalker is a fat girl that you can not fully wrap your arms around when you give them a hug.
Yo man i was at this party yesterday and Aunt Gertrude came... she is a Chalker man, i couldn't get my hands around past where her elbows would be.