1. That would depend on the context. A 'D-word' is a word that begins with D, and which you fear (or pretend to fear) to utter. A very pious person might refer to '****' as the 'D-word'. For Bill Gates, it could be 'DOS'. In most cases, it would be a humorous formation: a mock-euphemism.
The same applies to 'L-word'; except that in certain genres (would-be humo