Just asking questions? :dubious: 
This is a general approximation.
I think the whole concept would lose power if you ever actually could and did answer the OP’s question. The nebulousness of the concept is why it works. All you need to know is that the bogey man is very, very bad and horrible and scary.
Another interpretation told to me was it was originally “the Bonney man” meaning literally Napolian Bonaparte, used as a threat to children.
Meet Fungus the bogeyman!
Nah - he’s this guy: “He’s got supernatural rhythym, ten fingers on every hand, one big ear for music, known as the Boogie Man!”
I assumed at some point, once I’d become familiar with the mostly-Brit verb “to bugger”, that it had something to do with that. Buggery. You’ll get raped in the ass by depraved “bugger men” if you go out there in the dark etc.
Pratchett quote?
Need answer fast?
Oops, too late.
Keyser Söze is real tho, right?