Jim Grant thinks 'dad bods' could be a great reason to short Under Armour

Jim Grant has the next best idea on Under Armour.

And it is about "dad bods."

The dad bod look is a sort of middle ground between a man in good shape and one with a small beer belly and has become "popular" after a 19-year old college student wrote a "premiere dad bod 101" post that went viral. And according to market legend Jim Grant, this probably isn't goods news for athletic brands like Under Armour.

ValueWalk's Jacob Wolinsky tweeted what looks like an excerpt of Jim Grant's Interest Rate Observer, showing Grant laying out the thesis. 

Here's what Grant had to say:

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Trends seemingly come out of nowhere. Who can account for it? A UA short seller advises Lorenz that the "sneakerhead" market (an eBay-hosted emporium of athletic footwear) has "rolled over." A Clemson University student named Mackenzie Pearson writing in the Clemson Odyssey, coined the term "dad bod" and started a national celebration of the kind of guy who's not scared of a cheat meal, because he eats just about anything and everything." The untoned "dad body" is not Plank's core demographic.

With a $17.4 billion market cap, UA is a large, liquid stock. With a short interest of 7.8% of the float, it is not a crowded short. The insiders are selling $87.4 million's worth of stock at an average price of $76.23 in the year to date. Why shouldn't you?And here's that tweet:

Shares of Under Armour were lower but little changed in trading on Thursday morning.

The stock is up 45% over the past 12 months.