The informant attended a public high school in New York and first heard the phrase when he was a freshman (age 14).
Informant: I think some guy in the grade above us came up with it. And if he didn’t come up with it he definitely popularized it.
Collector: When would you use the phrase?
Informant: People could use it pretty meanly: If there was a nerdy kid or something. Or it could be used as a response to a “your mom” joke.
Collector: Could you give me another example?
Informant: Yeah. Let’s just say you and I are eating some sandwiches. You finish your sandwich and you go for a second and I say, “You’re having another one? You fatass.” But let’s say I had not only finished my first sandwich, I had already grabbed a second. You could say, “Coming from the kid.” […] Eventually people started shortening the phrase to just CFTK.
Collector: Did people text it to each other, too?
Informant: Oh, yeah. And people at the school still use it. My sister was a freshman when I was a senior in high school, and she still does it.
This example of folk speech illustrates that the need of a group to moderate the individual’s ability to retort to any potential verbal attacks, specifically pertaining to those that demonstrate a hypocrisy from an offender. Something of note is that the offender is diminished to a “kid.” This indicates that at least among the informant’s group of peers, a value is placed on maturity, that in high school, one is expected to grow up and leave behind childish behavior such as hypocrisy.
In this case, I found the speculated origin interesting. It appears that the student, who was a year above the informant at the time, has achieved a very minor legend status for his hand in the pervasion of the phrase. When asked to elaborate on the student, the informant suggested that everyone knew who he was because of his supply of marijuana, which made him the “cool guy.” So anything he said, people immediately adopted into their own vernacular. Whether this student’s status came from the invention/popularization of the folk speech or from his involvement illicit materials, it’s interesting to note that a speech phenomenon should be linked to a person and his social standing among peers.