Patricide is the crime of murdering your father. Like several other -cide words, it can also name the person who does it: a patricide is the killer, as well as the killing. It is a hard, formal word — law, history, tragedy on the stage. In ordinary talk people say 'he killed his father'. You will meet the pattern again and again: homicide (killing a person), suicide (killing yourself), matricide (killing your mother). The patri- piece is 'father', from Latin. It is not a word people stretch much, because the meaning is already so sharp; when it appears, it is meant to sound grave.