滾水 (Cantonese gwan2 seoi2) is used (often in restaurant kitchens) as a warning. It literally means "boiling water", but I think it indicates that something potentially dangerous or fragile will be coming by immediately.
Does it mean:
- Get out of the way (make room for me to pass).
- Don't move (I'll steer around you, but you must be predictable).
- Something else?
The first two require quite different responses, so it's important to know which is expected.