A 'Kill', or 'Kills' refers to the amount of Females or Males a person has had sexual intercourse with. Essentially their "Body Count" or "Kill Count" so to speak.
It's also a fairly more discrete way to let your
friends know how many Females or Males you've been with, without letting everyone know.
A
Female or Male can refer to their 'Kills' it's not single
gender oriented.