My dad used to call his trousers (not his underpants) “ratters”. I never did know where this word came from, but maybe it’s a word that ratcatchers use - trousers with string round the ankles to stop the rats going up their legs. I have a vivid imagination obviously! However, he wasn’t a ratcatcher. He was an army man for several years in his 20’s/30’s, including several years during WWII in the Far East and he had some cockney colleagues so maybe it was a cockney term?! He used a lot of cockney terms. Or maybe he did do a bit of rat catching out in the jungle?