To skim s.t. off of a liquid; to take s.t. out with a net attached to a pole (as fish from a pond).
When the -un imp. is used the object taken out is useful. When the -an imp. is used, the object is discarded unless the -an is beneficiary.
When you are making coconut oil, set it aside first for one night before you skim the oil off.
Who took fish out of my fishpond?