According to NGram Viewer, this is a new term first gaining some prominence in the late 1980’s. It would seem to mean based on what I can find, to reference a male homosexual relationship of some varying level of intimacy. However, its usage seems to be that the speaker is simply implying that two things or people appear to be highly bonded, so much so, that tongue-in-cheek, they appear to be lovers.

I see a reference to it in “Phrack Magazine” from 1993 in a suggestive, but not literal homosexual way “The software already present on the MCU is all that is needed, the interface need only support certain emulations (see Operation Procedures.) (hmmm… Could Radio Shack and Northern Telecom be butt buddies?)”. It’s not clear otherwise how the author intended the usage.

It appears in “ATTITUDE” #48 April 1998, but I don’t have this magazine and cannot give you the exact quote. Since this is a men’s gay magazine, it appears to be in reference to homosexuality, and maybe specifically male homosexuality.

The term ‘bum buddies’ appears to have been in usage about 10 years earlier and becoming more ubiquitous during the 80’s and 90’s before being abandoned in the mid 1990’s in favor of ‘butt buddies’.

Acronym	Definition
BBF	Back Button Focus (cameras)
BBF	Bad Boyfriend
BBF	Bockbüchsflinte (German shotgun)
BBF	Broadband Forum
BBF	Ball Bearing Fan
BBF	Building Bright Futures (various locations)
BBF	Blackbird, Fly (Japan, camera)
BBF	British Best Friend (Paris Hilton TV program)
BBF	Bumblefoot (band)
BBF	Brother’s Brother Foundation (Pittsburgh, PA)
BBF	Big Block Ford
BBF	Buffet Breakfast
BBF	British Baseball Federation
BBF	Border Book Festival (Las Cruces, New Mexico)
BBF	Bolsa Brasileira de Futuros (Spanish: Brazilian Futures Exchange; Brazil)
BBF	Box/Box/File (file cabinet under the work surface)
BBF	Bronco Bench Foundation (Santa Clara, CA)
BBF	Best Burgers Forever! (United Arab Emirates)
BBF	Betriebsbahnhof (German: Service Yard)
BBF	Black Bass France (est. 1994; fishing club)
BBF	Best Best Friend
BBF	Beer Buddies Forever
BBF	Better Boys Foundation (Chicago, IL)
BBF	Bursty Bulk Flow
BBF	Boys Before Flowers (Korean TV show)
BBF	Blood or Body Fluid
BBF	Best Boy Friend
BBF	Baptized by Fire (gaming clan)
BBF	Big Bang Fuzion (Micro-Star International; Taiwan)
BBF	Best Buds Forever
BBF	Belly Button Fluff
BBF	Burlington, Massachusetts (Airport Code)
BBF	Belgrade Beer Fest (Serbia)
BBF	Blinded by Faith (band; Canada)
BBF	Branded by Fire (youth conference)
BBF	Balance Brought Forward
BBF	Be Back in a Few
BBF	Big Blue Fox (artist)
BBF	Buis Blues Festival (France)
BBF	Burger Boy Food-O-Rama (Ohio fast food chain)
BBF	Bronchial Blood Flow
BBF	Best Bones Forever (US DHHS)
BBF	Business Base France (Paris, France)
BBF	Body Board France (online forum)
BBF	Brass Band Festival
BBF	Butt Buddies Forever
BBF	BigBrotherFans.org (online community and bittorrent tracker)
BBF	Blue Box Fraud (Sprint)
BBF	Bernard Bruche France (France)
BBF	Beyond Baseball Foundation (Canada)
BBF	Buzzerbeater France (online forum)
BBF	Bound by Fate (gaming clan)
BBF	Bongard-Bazot & Fils (French: Bongard-Bazot & Son; est. 1963)
BBF	Blackberry Friendly
BBF	Big Bang Fireworks (UK)
BBF	Bulletin des Bibliotechques de France (France)
BBF	Beautiful Bloody Fiction (band)
BBF	Bargain Box Fabrics (Australia)
BBF	Best Brother Forever (Internet slang)
BBF	Borden Burger Foods
BBF	Association Bernay-Burkina Faso (est. 1989; Burkina Faso)
BBF	Brain Stem Blood Flow
BBF	Bottom Billion Fund (charitable microfinance investment)
BBF	Baja By Fountain (Fountain Powerboat Industries)
BBF	Baseball Factory, Inc. (Columbia, MD)
BBF	Bridge Base Forums (gaming)
BBF	Berry Best Friend