马子 is crude slang for 'girlfriend' or 'woman,' common in street talk and gangster movies. It's disrespectful in most contexts — don't use it in polite company. Use 女朋友 (nǚpéngyou) instead.
usage
In some dialects, 马子 literally means 'chamber pot.' The slang meaning is separate and mainly used in northern Chinese and Taiwanese Mandarin.
