Trouty Mouth is an original song featured in Original Song, the sixteenth episode of Season Two. It is sung by Santana. 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.
Guppy Face, Trouty Mouth
Is that how people's lips look
Where you come from in the South?
Grouper Mouth, Froggy Lips
I love suckin' on those salamander lips
Wanna put a fish hook
In those lips, so cherry red
If you tried hard enough
You could suck a baby's head!
Oh, yeah-eah, yeah
Though originally unintended, Trouty Mouth was released on iTunes due to popular demand, along with Big Ass Heart.
This is the shortest song ever released of Glee, just over a minute and three seconds. Before its release, Papa Don't Preach was the shortest song.
Santana said she wrote another verse for the song, but does not perform it as both Sam and Will object.
In Born This Way, Sam admits that he is, or used to be insecure about his mouth through his T-shirt in the group performance.
Naya Rivera said in an interview that it turned really sexy. More sexy than she intended it to be.
The song was written by show composer James S. Levine and co-creator Ian Brennan. 
In the song Tina plays the piano (Jenna Ushkowitz learned to play piano for Spring Awakening).
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.