dark horse
US /dɑrk hɔrs/
UK /dɑrk hɔrs/
1.
a person who keeps their abilities and intentions secret and who surprises people by doing something unexpected, especially by winning a competition or election
:
•
No one expected him to win; he was a real dark horse in the race.
•
She's a bit of a dark horse; you never know what she's going to do next.