Trouty Mouth is an original song featured in Original Song, the sixteenth episode of Season Two. It is sung by Santana, in an attempt to declare her love for Sam, while also making fun of his large mouth. She is accompanied by Tina on the piano. Before being able to finish the song, Sam interrupts, offended by the constant mouth jokes. Santana writes this song with help from Tina.
After the New Directions discover that they are forbidden to sing My Chemical Romance's Sing, Rachel suggests that they should write original songs for Regionals as it will increase their chances of winning. Santana protests when Rachel and Quinn offer to write one, saying that all of them should have the chance to write their own songs. Thus, Will assigns them to write original songs.
After Will rejects Hell to the No as a song for Regionals, Santana again offers "Trouty Mouth," saying she has written another verse for the song (though Sam objects by holding up a "Hell No" sign). The additional verse was not performed, or included, in the version released on iTunes.
This song is referenced in Naked, the twelfth episode of the fourth season. In Blaine's documentary for Sam, Santana's testimonial was about Sam helping her become a songwriter. She then sang some lines of the song.
From that song and on, many people use the phrase "Trouty Mouth" to describe Sam and his lips; especially Santana.
Sam used "Trouty Mouth" as his synchronized swim nickname in the Season Three episode Yes/No.