Dictionary.net

(From the 1930 Sellar & Yeatman parody "1066 And AllThat") Something that can't possibly result in improvement ofthe subject. This term is always capitalised, as in"Replacing all of the 9600-baud modems with bicycle courierswould be a Bad Thing".Opposite: Good Thing.British correspondents confirm that Bad Thing and GoodThing (and probably therefore Right Thing and WrongThing) come from the book referenced in the etymology, whichdiscusses rulers who were Good Kings but Bad Things. This hasapparently created a mainstream idiom on the British side ofthe pond.