Abraham Lincoln was first labeled "Honest Abe" when he was a young store clerk who went out of his way to ensure customers were not shortchanged. Lincoln's honest nature led to many calling on him to help mediate disputes.
As a lawyer, Lincoln was said to have trouble arguing cases in which there was a morality issue. His honest nature extended to his personal relationships, including with his wife. Historians have found no evidence that Lincoln ever had an affair.
When Lincoln became president, his honesty helped him to negotiate difficult situations, including the issue of slavery. He also was upfront with his staff about their work ethic and character.