it used to be "by-lounge", but it was shortened in Victorian times to b'longe... it's for describing that something lounges by to someone
Of is one of those abbreviation-turned-words : O.F. was a widespread shorthand for "out from" before the War of the Roses, which during that time turned into a word in itself.
That's why you can always replace "of" with "out from" and get something interesting. Or perhaps that's a matter out from taste.
Disclaimer: do not repeat these definitions. I grabbed them straight out from my ass. If you do repeat them, wear gloves