see a man about a dog idiom UK informal
出去一下（尤指上厕所的隐讳说法）
You might say you have to see a man about a dog when you do not want to say what you are really doing, especially when going to the toilet.
I've just got to see a man about a dog. I'll be back in a minute. 我出去一下，一会儿就回来。