A name for the theoretical extra day you worked when you've already worked seven shifts during the week.
Also, a name for a day on which you work two or more shifts during a week in which you'll be working more than seven shifts.
It's a combination of Thursday, which is often payday, making a Thursnack slightly more bearable, and knackered, because that's what you are at the end of the week.
Employee 1: What do you work this week?
Employee 2: Monday to Sunday, with a Thursnack on Saturday.
Employee 1: Man, we seriously need to hire some new staff.