The hardest thing in (the) English language

English is not my native language, but I’ve become pretty good at it. This one thing though…

Check out this quote by Eminem (the song “The Ringer” from the album Kamikaze):

“Can get a mouthful of flesh
And, yes, I mean eating a penis”

So it’s a penis here. Note the article. Eminem is not talking about some specific penis, but rather a non-specific, non-particular penis.

On the other hand, I also see the expression “suck cock” a lot.