1)The act in spending a particular amount of time with someone(s) and engaging in some physical intimacy.
2) Killing, is fooling around, time is time, thus fooling around during that time.
3) Just like hooking up, but it's killing time.
4) killing time can mean, making out, oral sex, sex, anal sex, orgies, swingers parties, milf adventures, dilf adventures, sexual exploration, homoerotic endeavors, lesbian's curiosities filled, etc.
Byron- Hey man, how was last night?
Devin- It was fun man.
Byron- What did you do?
Devin- I was killing time with Laura.
Hey Tina, you wanna get together and kill some time?