Play it cool
Definition 1 of 2
Pro Tip 1/3Emotional Restraint
Use 'play it cool' to suggest controlling emotions, even if you're anxious or excited. 
He won the award but played it cool in front of his colleagues.
Pro Tip 2/3Understated Reaction
It implies giving a muted response to an event that would normally elicit a stronger one. 
When she mentioned the rumor, he just played it cool.
Pro Tip 3/3Masking Nerves
Use the phrase to describe hiding nervousness in situations where appearing confident is beneficial. 
On his first day, he played it cool even though he felt lost.