Evidently it means immediate review, according to Word Detective it comes from the US Army where it describes the debrief that occurs immediately after a mission or patrol, possibly from the literal talking while showering, more likely from the practice soldiers have of dousing their weapons in hot water after an exercise.
I’d never seen it before, and nor had any other colleagues in the room. So I’ve asked people what it makes them think of, most people said either laundry or hot tubs (which might say more about them than the subject). But the most descriptive was “hotwash sounds like a painful spa treatment involving large muscular women twisting your body in weird ways”.
In this case it was referring to the last hour of an assessment day, when all teams will discuss their assessments and we’ll check any major inconsistencies.
If I’d been writing the agenda I would have put “4 – 5 pm Review Assessments”, but then, I’m not a management consultant.