Lunkhead was an insulting nickname that was meant in jest rather than cruelty.
In 2367, Data called Geordi La Forge a lunkhead after the chief engineer questioned if Data was visiting USS Enterprise-D's salon for a haircut. He explained to a confused La Forge that he was experimenting with friendly insults and didn't mean to hurt him. Geordi suggested Data not try insulting Captain Picard thusly.
Data related this incident in a letter addressed to Commander Bruce Maddox, which was intended to describe a typical day in the life of the Soong-type android. The letter detailed these events with a focus on Data's observations regarding friendship. (TNG: "Data's Day")