The idea of a marinade is that you slop something vaguely runny all over something more solid, and then you leave it for a while so that the something solid soaks up the runny stuff.
A marinade was originally used for food, especially meat and fish, but nowadays it's not unknown for people to marinate themselves in sunscreen or moisturiser.
(I really don't think this lady is a cannibal.)
(It's got to be something that soaks in, or is supposed to soak in - you can't marinate yourself in water, or mud.)
Small children have a natural urge to try to marinate themselves in anything they can get their hands on, but this will only be tolerated happily by the most insanely besotted of parents.
I've just said that you can't marinate yourself in water. This is odd, because the word marinate looks very much as if it has some connection with the sea.
And it has, too.
Thing To Do Today: marinade something. This word came to English in the 1600s from France, and before that from the Spanish marinado, to pickle in brine. Before that it goes back to the Latin marīnus, from mare, which means sea.