Originally “Bogdan's Erlang Abstract Machine”, but then the subsequent maintainer was named Björn so you could sub that in too.
And the predecessor to BEAM was JAM (Joe’s) which is a bit of a shame we didn’t end up with that acronym… or not, I guess.
IIRC Joe Armstrong said he was puzzled why people latched on to the term "BEAM" so much, and he preferred that it was simply referred to as the Erlang VM.
Beam, with one syllable, is more convenient to say out loud than Erlang VM. It also sounds cooler.