Wikipedia:Ad nauseam
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 | This page in a nutshell: Ad nauseam is spelt ad nauseam, not ad nauseum. |
At the time of writing, August 2013, Wikipedia contains around 3,100 instances of ad nauseam, and about 4,100 instances of ad nauseum.
This seems like a good time to point out that the correct spelling of the Latin phrase is the one that is rarer in our project – ad nauseam. The literal meaning of the phrase is, "to [the point of] sickness". Ad is Latin for "to", a preposition followed by the accusative case, and nausea, meaning "sickness", is the Latin word that has given us our familiar "nausea" in English. Nauseam is simply the accusative case of nausea.
So, please take note of this, so we don't go on repeating the erroneous spelling ad nauseum ad nauseam!