means a person who exposes wrongdoing, corruption, or illegal activity within an organization, usually from the inside and often at personal risk.

from From the literal act of blowing a whistle to call a halt or raise an alarm — think of a referee stopping foul play, or a police officer signaling a crime in progress. The figurative sense of 'one who sounds the alarm on misconduct' took hold over the 20th century, with the compound 'whistleblower' becoming common in American English. A popular tale ties it specifically to bobbies' whistles, but the image draws on the whole family of alarm-whistles: sports officials, factory floors, and the law alike.