[john-aj]

Yeah, I think it is more likely that it is related to German "(be)schließen", Swedish "(be)sluta" etc. which originally means "close, shut, join together, lock up" etc.

OTOH, that verb usually corresponds to Dutch "sluiten". "(be)slissen" seems to mean the same as "(be)sluiten", but with a different etymology. I'm not sure whether it fits chronologically, but perhaps it is a loan from German or a development, atypical for Dutch, but similar to that in German?

[Blikkentrekker]

In practice the verb is suppletive with it's synonym “besluiten”. In theory both can be used interchangeably but there seems to be a strong favor to use “beslissen” in the præsent imperfect, and “besluiten” in all other tenses and “aspects”.