Someone slaphappy is cheerful in a loose, silly way, and that looseness makes mistakes likely. They joke, they rush, they do not stop to think — and something breaks. It is informal, a little old-fashioned in some places, and more American than British. Though actually, it is not the same as happy. Happy is a feeling. Slaphappy is the smile plus the carelessness: the mood is light, the work is not. An older, darker use is for a boxer who has been hit too much and acts dazed and silly — close to punch-drunk. The meaning you will meet now is the giddy, careless one.