sleeper

sleeper /ˈsliːpɚ/ noun
plural sleepers
sleeper
/ˈsliːpɚ/
noun
plural sleepers
Britannica Dictionary definition of SLEEPER
[count]
1 
a  : a person who sleeps in a particular way
b  : someone who is asleep
2 
a  : a place or piece of furniture that can be used for sleeping — usually used before another noun
b  : sleeping car
3 
chiefly US, informal : someone or something that suddenly becomes successful in a way that was not expected
4 
: a spy who lives and works for a long time like an ordinary person among the people he or she is spying on — often used before another noun
5 
US : a piece of clothing that covers the entire body including usually the feet and that is worn by a baby or small child
6 
British : one of the heavy beams to which the rails of a railway are attached : tie