unflappable ˌən-ˈfla-pə-bəl adjective
: not easily perturbed, excited, or upset; marked by extreme calm and composure
The word unflappable has appeared in 73 articles on NYTimes.com in the past year, including on Jan. 22 in “Eli Manning Announces His Retirement” by Bill Pennington:
A brother of Peyton Manning, who was one of the most productive and celebrated quarterbacks in pro football history, Eli Manning was the quiet, younger sibling who instead became known for sterling performances in the biggest games of his career. Durable and unflappable, he also had an unassuming, workmanlike demeanor that played well during a lengthy career in the country’s largest media market.
… “In the most pressure-filled, intensely competitive situations, that’s when you knew you could count on Eli — and boy, he would never let you down,” Tom Coughlin, who was the Giants’ coach during both Super Bowl-championship seasons, said in 2014. “That will forever be his calling card.”