Old joke.
Back in the day, a young abolitionist tries to join the Union Army. The recruiting sergeant laughs at the puny lad. "Come back when you've got some hair on your chest, milksop!"
Walking home, the lad passes President Lincoln. The dejected lad pleads his case to the President. Lincoln looks at the boy and asks, "Do you have an older sister?" The boy says he does.
"Tell your sister to remove her undergarments and sit on your chest every night for a week."
The lad is stupefied by the bizarre pronouncement, but goes home and puts the proposition to his sister. She is equally mystified, but acceeds to the strange request.
Lo and behold, not a week passes and the lad has as hairy a chest as you'd find anywhere. He returns the the sergeant, and that worthy allows the young man to join the Army of the Potomac.
After displaying uncommon valor in battle, the now corporal is sent to Washington to recieve a medal from the President. Lincoln recognizes his protege and invites him to tea.
As they chat the soldier asks the President. "Sir, how did you know that having my sister rub her privates on my chest would cause hair to grow there?"
"You must let me keep some secrets," said Lincoln as he stroked his beard.