A cokehead is a slang, often unkind name for a person who uses the drug cocaine a lot, or who is hooked on it. Coke here is the street short form of cocaine, and -head is the same ending you hear in words like crackhead: it paints someone as that kind of heavy user. It is informal, and it can sting. Films and news use it; you should not use it about a real person standing in front of you. In careful writing, cocaine user or a person with a cocaine addiction is kinder and clearer. Though actually, coke in other rooms of English can mean Coca-Cola or the hard fuel made from coal — here it is only the drug.