Abraham Simpson is the son of Mabel Simpson and Virgil Simpson. Abraham was himself almost definitely named after Abraham Lincoln who directly helped Virgil and Mabel escape from pursuing slaveholders. Abraham is also the great grandfather of Abraham "Grampa" Simpson and presumably his namesake.
When Mabel and Virgil got married, they had a son and named him after the president at the time, Abraham Lincoln. He is only seen in brief moment wearing Lincoln's hat he gave to them. He strongly resembles Bart Simpson, but with ruffled hair.