The act of drinking with the intention of achieving moderate alcohol intoxication before moving on to a formal establishment - such as a pub or night club - where drinking will often continue to ultimately achieve the status of being 'steamed' (extremely drunk).
James:"Hey Matt, I think it's about time we got on with presteaming as it's getting a little late, don't you think?"
Matt: "Nah mate, we've still got time to fit in a good session
, and then we'll be well on our way to being steamed
by the time we hit the pub."