A slaphead is usually a guy who is so naturally bald he makes most bald people look positively hirsuit, but really it can be any guy who is definitely bald or going bald. A slaphead is so obviously bald that you just wish it was acceptable to point out the patently obvious by slapping them 5 or 6 times in rapid succession on their bald shiny head. The term slaphead was actually introduced in the comedy of Benny Hill, a world famous British comedian (deceased). Benny always used a sketch where a small bald man would do something a little annoying to Benny Hill and benny would slap the guy a few times on his bald head.

A term used to reference cannabis/marijuana/weed.

A bald person, or in the absence of a bald person, someone with a large forehead

an absolute baldy!

A person with a bald head. Usually one that looks as if it's been waxed.

Slaphead can affectionately point out something stupid done by yourself or a colleague. Its that moment when you realise the problem you have spent ages trying to find is discovered and its soo simple or soo stupid and soo obvious.
Its that point whn you mentally hit yourself and say something like D'oh

someone who is soooo stupid you have to slap them in the head