To have doggy style sex with your partner as if riding a horse.
Not sexuality specific, strap-ons acceptable
Spanking is also not only acceptable but encouraged
Seabiscuit refers to the horse-racing film starring Tobey Maguire
Jack: How did you get on with Sally last night?
Dave: It was awesome, I managed to ride her like Seabiscuit!!
Jack gives Dave a high five
Shortened form of 'designated driver'
A person in a group of mates who has been chosen to drive to the pub/club/party and thus has to stay sober for the whole night.
Because of this he/she - as payment - has all drinks bought for him/her for that night
Jack: Who's driving to Sally's partay tonight?
Dave: Des is gonna be des
Jack: What Des is the des?
Jack: Fair enough...