1. to behave in a way that is not genuine, authentic, or characteristic of one's normal personality; to behave affectedly or artificially
She was acting unnaturally around her new boyfriend, trying too hard to impress him.
Ela estava agindo de forma antinatural perto do seu novo namorado, tentando muito impressioná-lo.
2. to perform actions that go against one's nature or instincts
He was acting unnaturally by suppressing his true feelings.
Ele estava agindo de forma antinatural ao reprimir seus verdadeiros sentimentos.