There was a specific word in the script that was used for the recording. The word was chicken but could have been many other words that would have achieved the same result.

 It was supposed to be "**** off" and "chicken" gave the same mouth positions in animation. In animation the dialog is recorded before animation and it was animated in a non English speaking country. Korea in this case.
The process is like this. Write the script and record the dialog. The storyboard artist is told that the word "chicken" actually means '**** you" in this show so they aren't confused.

Then the voice recording is broken down phonetically frame by frame on the animation exposure sheet. An animator in Montreal that speaks English assigns one of 6 mouth positions for every frame of the show based  on the phonetic breakdown. THE six mouth positions are numbered A,B,C,D,E,F.
' A ' being a closed mouth for "m" , "b", "p" sounds, B mouth for most consants, C mouth for most vowels except "00" and "OA" sounds  and 'F' is for "oo" or "r" sounds. 

In this case to say "**** off" you would use the mouths in the sequence  B,C,B,C,B. Also to say "chicken" you would use B,C,B,C,B.

When the Korean animator does his animation he follows the appropriate mouth position that is written on the exposure sheet by the Montreal animator and  that he gets the correct mouth position from the model sheet. The animator doesn't even need to know the meaning of the words.

It's the same mouth system used at Hanna Baraberra starting from the Flintstones era. Some animation uses a different system that has  2 extra mouths to make a sound "LLL" and one for the "V" and "F" sounds. Arthur uses the 6 mouth system not the 8 mouth system. If Arthur used the 8 mouth system there would have been different mouth positions on the "f" sounds.

We could have just used the word '**** off' in the recording since we were going to bleep it anyway but then the script and the recording would have have the swear word in it, and that would have made the clients WGBH  and Marc Brown too nervous during the process.