To grab, buy, or do something before it is necessary to do it in order to prevent having to do it in the future — just in case it would be necessary to grab, buy, or do it later.
Imma pre jack that PS5 a month before Christmas, in case they sell out at that time.
shortened term for pre-ejaculatefluid, the filmy substance that contains sperm, ova, etc. that leaks out the penis or vagina before the orgasm during sex.
"I dunno how my bitch got pregnant! I pulled out and shot my load on her face. It must have been the prejack!"