be a one idiom UK old-fashioned informal
是个活宝；是个有点无礼的人
to be funny in a slightly rude way or in a way that shows no respect
"He told me I couldn't have the job and I told him I never really wanted it in the first place." "Ooh, you are a one." “他告诉我不可能得到那份工作，我对他说我压根儿就不稀罕。”“噢，你可真放肆。”