a) (for people between the ages of 18-25): having sex with your significant other for the sole purpose of placating either your or the sigother's parents by giving them grandchildren to play with
b) (semi-offensive): losing one's virginity in a one night stand
"Ignition" is, among other things, the slang word for "vagina". A "penis" is called a "key", because you stick the penis in the vagina like you stick a key in the ignition
R. Kelly sings: "Let me stick my key in your Ignition, baby"
(person 1)lets go to the park
(person 2)yeah sure dude just let me smoke some pot first
(person 1) wow man, i see your starting that herbal ignition stuff.