What is the context, please?

Without context, it's difficult to tell. Two things come to mind: it makes it shiny or it makes it slippery. I've never heard the expression before, but we do hear 'like water off a duck's back', meaning that is just doesn't stay.

I'm guessing this means an effect similar to when water splashes on a stone. When this happens there is no lasting effect, the water just washes off. Water can have an effect on a stone's surface but only after many years. So this would mean that something makes no lasting impression, and for an impression to be made would take a long time. For example:

He's shy and inarticulate, and