know only too well
To be very familiar with something, usually something bad or unpleasant. A noun or pronoun can be used between "know" and "only." Oh, I know sleep deprivation only too well, what with three toddlers at home. We know only too well how stressful moving can be.
know something only too well
to know something very well; to know something from unpleasant experience. (Note the variation in the examples.) I know the problem only too well. I know only too well the kind of problem you must face.
know only too well
see under only too.