Rehoboam's young advisers said his little finger was thicker than his father's what?

Answer: Waist

The young men advised him to say, 'My little finger is thicker than my father's waist!' and to threaten scorpions instead of whips.

“The young men who had grown up with him replied, “This is how you should answer these people who said to you, ‘Your father made our yoke heavy, but you must make it lighter.’ This is what you should tell them: ‘My little finger is thicker than my father’s waist!”— 1 Kings 12:10 (BSB)