As <PERSON> said, "B.S." stands for "<PERSON>" in the second Swedish translation of Lord of the Rings, based on the translation "bag" -> "säck" (pronounced "seck"). While the word "bag" exists in modern Swedish, it's an obvious direct import from English and does not work in context.
There is however a bit more depth to this story. The original Swedish translator of the Lord of the Rings trilogy, <PERSON>, was a bit of a special character. 169
He took great liberties with his translations, and would both embellish the language and try to figure out the origins of a name to recreate it in Swedish rather than translate it. This made his translations quite popular among the readers, but his rushed work, agnostic view of proof-reading and sometimes spotty knowledge of English would often lead to more or less glaring mistranslations. He also didn't take kindly to any form of criticism.
These mistranslations caught <PERSON>'s eye, as language was very important to him (being a philologist and all), and would lead to an escalating conflict between the <PERSON> family and <PERSON> over the years. This lead <PERSON> to produce a translation guide for his books to make sure the mistake would never be repeated again, and when Swedish publisher Norstedt approached the <PERSON> family to ask for permission to do a second translation, <PERSON> only allowed it on condition that it would have nothing to do with <PERSON>' translation.
The second Swedish translator, <PERSON>, employed a group of Swedish <PERSON> fans to review his translation and provide suggestions, but his faithfulness to <PERSON>'s laconic prose over <PERSON> more poetic translations combined with the fact that a lot of names had to be changed to follow the translation guidelines (<PERSON>'s and <PERSON>'s surname being a prime example) means many Swedish fans still prefer <PERSON>' translation to this day.
To return to the original name, I personally don't believe <PERSON> mistook the root of "<PERSON>" for "bagge" (ram, male sheep) in his translation, but rather that he didn't think the name meant anything at all and simply changed the ending to roll better off the tongue in Swedish. 169