noun. sing. (plur. preparties)
Before going out to a club or concert where you know the alcohol is going to be expensive, you meet up at someone's house to have a few drinks, usually in a short period of time, so that you arrive at the event properly buzzed, or even somewhat tanked.

verb. to preparty

The action of partaking in a preparty.

When you eat dirt for the first time and leave your family to build a shack in the woods

The party that comes before the main event.

To have an afterparty before the actual party begins.

Preparties usually involve getting drunk and preparing oneself for the party to come. By the time the people get to the party they are already messed up and having fun.

Preparties often turn into small parties themselves.

The preparty is the appetizer to the main course.

It can be used as a verb or a noun.
Verb: the act of going to a preparty, or participating in a preparty.
Noun: the preparty itself, the event before the party

to imbibe alcohol before an official function