The chick you fuck when the hot girl wont let you. The easier uglier one. Usually you won't touch it at all unless you're desperate that night or forced into it.
The hotter girl, the one you hit on first, may reject you after trying to get in her pants. Then you're told you can get it from the easier homlier one and you try to get with the hot one again 15 times then are left with the ugly one wanting your dick instead.
The hot girl gets you all worked up and won't fuck you and the uglier easier one is sitting right next to her willing to give it up.
This is the last option. The question is do you do it or not?
Marty: Yo Sean, you fuck Amy last night?
Sean: No, she wouldn't let me and insist I fuck her grenade friend instead.
Marty: Damn dude, were you that persistant?
Sean: Yes, but she would actually be worth it to fuck, Instead I was stuck with Sarah after being forced by her friend as a pity fuck. I sure wish i had a wingman that night. That's ok though, i thought of the hot one but had to take the last option.
Marty: Good thinking.