the bathroom with the lest traffic in any building (whether because of location, facilities or cleanliness) where considerent people go when what they have to do could be unpleasant for others. Example: I think I'm coming down with the flu or something -- I'd better use the second bestroom.
SECOND BESTROOM
Meaning of SECOND BESTROOM in English
Slang English vocab. Английский сленговый словарь. 2012