Bad Thing: [from the 1930 Sellar & Yeatman parody "1066 And All That"] n.

:Bad Thing: [from the 1930 Sellar & Yeatman parody "1066 And
All That"] n. Something that can't possibly result in improvement
of the subject. This term is always capitalized, as in "Replacing
all of the 9600-baud modems with bicycle couriers would be a Bad
Thing". Oppose {Good Thing}. British correspondents confirm
that {Bad Thing} and {Good Thing} (and prob. therefore {Right
Thing} and {Wrong Thing}) come from the book referenced in the
etymology, which discusses rulers who were Good Kings but Bad
Things. This has apparently created a mainstream idiom on the
British side of the pond.
-- The AI Hackers Dictionary

Related:

• Bad Thing n. [very common; from the 1930 Sellar &
  Yeatman parody "1066 And All That"] Something that...
• Good Thing: n.,adj. Often capitalized; always pronounced as if capitalized.
  1. Self-evidently wonderful to anyone in a position...
• Good Thing n.,adj. [very common; often capitalized;
  always pronounced as if capitalized.] 1. Self-evidently...
• Wrong Thing: n. A design, action, or decision that is clearly incorrect or inappropriate.
  Often capitalized; always emphasized in speech as...
• Right Thing: n. That which is *compellingly* the correct or appropriate thing to use,
  do, say, etc. Often capitalized, always emphasized...
• Bad and Wrong adj. [Durham, UK] Said of something that is both badly designed and wrongly executed.
  This common term is the prototype of, and is used...
• trap door: alt. `trapdoor' n. 1. Syn. {back door} -
  a {Bad Thing}. 2. [techspeak] A `trap-door function'...
• In life there is but one bad thing and one good;
  both of them are women...
• Wrong Thing n. A design, action, or decision that is clearly incorrect or inappropriate.
  Often capitalized; always emphasized in speech as...