What does the expression "孤男寡女" mean?
Frankie's answer is good, but I want to make a clarification on 孤 and 寡. Both of them mean &quot;only one&quot; here, and I don't think &quot;single&quot; is good in this situation since it could mean &quot;unmarried&quot;.
example:
you and a female friend of yours are taking in a  club, in a room with the door closed,(of course, I don't recommend this :-)  ), unfortunately, your wife finds you here, so she would high suspect that you have affairs with your friend.You try to explain, but she retorts,

你们孤男寡女，共处(chŭ, to stay in )一室，你还有什么好说的？
one man, one woman, and one room, how could you explain?(I won't believe!)