It comes from a line often attributed to the 19th-century humorist Mark Twain: Eat a live frog first thing in the morning, and nothing worse will happen to you that day. ”Eat a frog” is intended as a metaphor for “do that shitty task you don’t want to do”.

It appears to have actually been coined by a late 18th century French humorist: https://quoteinvestigator.com/2013/04/03/eat-frog/