To go out to the pub or a restaurant directly from work on site without washing and changing clothes first. This may mean that you stand out from the crowd in your snickers workwear, high-vis vest and safety boots. Not usually advisable unless you dont mind looking like a total twat.
Engineer 1: "Lads, it's 19:45 - we need to wrap this shit up and get some dinner. Will we go back to the hotel and meet up in the bar?"
Engineer 2: "OK, but let's go out on a black 'un - I can't be arsed to go back and get ready to go out".