J.R. Clark-Hall's Concise Anglo-Saxon Dictionary defines understandan as to understand, perceive, observe, notice, take for granted, which fits in (sort of) with tsuwm's range in his post. Clark-Hall also references the Latin subsistere leading me to suppose that it was taken as a loan-translation from the Latin, but according to tsuwm's etymology citation this must have happened well before English hit the British Isles.