Pap Finn is Huckleberry Finn's abusive, alcoholic father.
After a long disappearance, during which time he was supposed to have drowned, he appears in the Adventures of Huckleberry Finn, interested in laying a claim to his son's riches. Huck gives the money to Judge Thatcher, and Pap holds him captive in a remote cabin while he tries to get it back through court action. Huck fakes his own murder and escapes; Pap falls for the elaborately fabricated crime scene and seeks help, but soon gives way to his own drunkenness again.
In the closing pages of that book, Jim reveals to Huck that Pap Finn was the dead man they had found in a ransacked house floating down the river some distance back, giving Huck the peace of mind to return home to St. Petersburg, adopted by Aunt Sally at the cost of being "sivilized".