The man was nonchalant
She was talking nonchalantly to her mom.
His heart pounded in his chest, and he tried to look nonchalant.
You could say: Nathan's heart was beating loudly and feared his girlfriend would hear it, however he continued to act nonchalant.
How can you be so nonchalant about such an important job interview?He was nervous about asking her out, but he tried to seem nonchalant.The judge seemed annoyed by Lindsay Lohan's nonchalant attitude...