DUHHHH

SHMAMDAYSHANWICHES
SH-MAM-DAY-SHANWICHES PROLONGED
SHMAMDAYSHANWICHES
SHMAMDAYSHANWICHES
SHMAMDAYSHANWICHES
SHMAMDAYSHANWICHES
SHMAMDAYSHANWICHES
SHMAMDAYSHANWICHES
SHMAMDAYSHANWICHES
SHMAMDAYSHANWICHES

Now say it 10 times.

Now what’s the first thing that you think about after you said
it 10 times?

Repeat 3 more times and asnwer this question: What is a
SHMAMDAYSHANWICHES?

Figure it out and you will get the joke, it’s easy.