Chances are, you’ve sported a T-shirt in the last couple of days. But did you sing “Happy Birthday” to it?

Yes, this year is being trumpeted as the 100th birthday of the much-beloved, almost-everyone-has-one T-shirt.

But wait a sec. While it seems to be fact that the U.S. Navy introduced sailors to a “light undershirt” in 1913 (the poor guys had been sweating it out in itchy wool), their European counterparts had already donned the lighter shirts as summer undergarments.

So the T-shirt — so named because its shape resembles the letter T, natch — has to be older than 100....