What is Unflappable?

Dave Flournoy.

I hit Dave Flournoy, and he kept singing to the birds. They flapped away, but he was unflappable.

Unflappable - meme gif

Someone who is not easily disturbed or deterred, and will typically excel under pressure/have calmness in a crisis.

The college scout knew she was unflappable when she kept her composure, and lead the team to victory in the final seconds of the big game.