Means to stop working or doing whatever you’re doing because you’ve had enough, you’re knackered, or you’ve decided the effort is no longer worth it. Can be sensible, like finishing a proper shift, or wildly optimistic, like clocking off after ten minutes. Often said with a sigh, then straight to the kettle.
"Right, we’ll call it a day then. It’s 2pm. Yeah and I’ve mentally done a full shift, mate. Put the kettle on, I’m grabbing my coat"