The foul tongue is a sex move where both participants perform cunnilingus on each other's rectums, then proceed to French kiss.
If there is feces on either participants tongue, the sex move transfers into "The tarnished tongue".
"Jeffrey, might I say, I was having the most lovely time performing cunnilingus on Sarah the other night, but she asked me to perform 'The Foul Tongue'! I felt sick to my stomach so I couldn't...quite unfortunate"
"Ah quite unfortunate Robert, however you see, I performed it the other night on Abbigail and she had brought feces up from my rectum! Quite 'The Tarnished tongue' I had experienced."