Narrator: If I did have a tumor, I'd name it Marla. Marla. The scratch on the roof of your mouth that would heal if you could stop tonguing it, but you can't.
What does "tonguing it" mean here? This dictionary says it refers to musical but here context is not at all musical.
Source: Fight Club (1999)
The fact that he refers to the scratch as being "on the roof of your mouth" is your hint that we're speaking literally here: "tonguing it" just means touching it with your tongue. (You can see that definition on the dictionary.com page you linked if you scroll nearly all the way to the bottom.)
"Tonguing" is also slang for French kissing, and that meaning may be why the narrator chose the particular phrasing of "tonguing it" instead of just "touching it with your tongue". There's a little more dramatic effect with the "tonguing it" phrasing.
