From Longman Dictionary of Contemporary Englishboys will be boysboys will be boysBEHAVEused to say that you should not be surprised when boys or men behave badly, are noisy etc → boy
Examples from the Corpus
boys will be boys• Boys will be boys and in spite of warnings at least one fatal accident occurred involving local lads, and several were injured.• Above: Boys will be boys.• That, and a few water spouts, but then boys will be boys. 