I think the word 'bejeezus' stems from the phrase, 'by Jesus' and is basically an intensifier. Like if I said: "You scared the holy bajeezus out of me." That's a much more expressive sentence than, 'Gee, you scared me." This bastardization of words is similar to how Dad Gum evolved from God Damn and biznitch or biotch from bitch. Slang origin is neat-o, yo.

sidenote: I am so in love with the fact that the howling fantods is softlinked here.