Second that emotion was the title of a popular song by Smokey Robinson in 1967. It was a play on words from the phrase second that motion. It can be used to say that someone doesn’t feel the same way as someone else, but it is almost exclusively tongue in cheek and referencing the song.
Second that motion is a phrase used in formal meetings. A motion is a proposal of action, which may or may not need to be seconded. To second something is to agree that it should be done. Usually a motion needs to be seconded before it can be put to a vote from the entire group.
By saying you second that motion, you are saying that you agree with the proposed course of action or that you agree with the idea.
Over time the mondegreen second that notion has appeared. Most of the time speakers may use the phrase without knowing, or caring, whether they are saying motion or notion. By far motion is more common and is deemed the official phrase, even though the mondegreen does make logical sense.
Examples
The first person to second that motion was right tackle Troy Baker, a fifth-year senior and vocal team leader. [Star Telegram]
Seconding that notion, Niall Horan, 21, who was last to take to social media following Zayn’s departure, also focused solely on the band’s sound. [Daily Mail]
Reps. Kathy Sims and Ron Mendive, both R-Coeur d’Alene, don’t second that emotion. [The Spokesman Review]