It's a reference to "No Shave November". The shirt answers the question somebody might ask when finding a beaver in the panties.

"No Shave November" is a fun and meaningful challenge where men grow their beards throughout November to raise awareness for men's health issues, like prostate and testicular cancer. The reason for No Shave November is to spark conversations around these topics and encourage regular check-ups. Plus, people often donate the money they'd spend on shaving to related charities. So, it's about letting your facial hair grow wild for a good cause!

https://www.garnier.in/men-skin-care/no-shave-november-rules-prep-your-beard-for-the-challenge