There are actually several familynames like that, bockelman, bogeyman, bogchelman
Another possible explanation is bockeman as in buck. Like half man, half goat, ring any bells?
The third option is boe(s)man buschemann. meaning bushman. I seriously remember being threatened with that when I was young, again not very politically correct. But with the same aspect something unknown and different than average. (When I was really young I thought that meant men hiding in little shrubs..)
So you cooould say, if it does not have a hump/is hunched, is not from the bush, or is not half goat, it is not a bogeyman ;)
Hope I explained it well enough in english. Hard to make descriptions in another language, especially in text. But if you didnt know this already, educate yourself and research it.