Ad nauseam is a Latin term used to describe an argument which has been continuing "to [the point of] nausea". For example, the sentence, "This topic has been discussed ad nauseam", signifies that the topic in question has been discussed extensively, and that those involved in the discussion have grown tired of it.
Many will use this argument fallaciously to try and dismiss certain arguments they don't agree with, saying, "This has been discussed so many times and no one cares.. It's simply not valid anymore!" without providing any evidence that the argument in question is actually an insufficient argument.
Often times used in conjunction with the Argumentum ad lapidem