These days a large percentage of clothing has words (often advertising) on it. But, what was the first time this was used?
The first “slogan t-shirt” was supposedly created for Thomas Dewey’s 1948 presidential campaign and was emblazoned with the words Dew It With Dewey.
Levi’s first put the “red tab” Levi’s mark on the back pocket of their jeans in 1936 specifically to separate them from competitors.
I’m pretty confident we’ve been slapping words on apparel for as long as we’ve been writing.
Baseball uniforms had team names and cities on them back in the 1880s.
Letterman jackets were first created at Harvard in 1865, specific to achievement for baseball players, and quickly more schools and more sports began using them.
It seems like the letterman sweater was pretty popular in the 1920s and 30s, but just for men.
But as far as I can tell from examples, they were always just the school name letters or your grad year.
Not sure if that really fits the OP’s question tho (nor do sports uniforms or fan sports apparel, IMHO), because while the sweaters weren’t uniforms they were still given out by the teams and clubs and not for sale to the general public. I’m pretty sure @beowulff is looking for slogans or brand names.
What was the first time someone wrote the days of the week on their underwear?
What like… Monday forward, Tuesday backwards, Wednesday inside out, Thursday inside out and backwards?
That’s better than the 12 pairs of underwear system.
I’m going with prison clothing. If numbers are included.
I just so happy I don’t see young ladies with big block letters spelling PINK on their butts anymore. That was asinine. 
(Didn’t very early British navy have letters on their sailor hat thingys…oh do we include embroidered monogrammed clothing? That’s been around a long long time)
Just a thought: What about priestly robes? Are there religions that have letters or characters on priests’ garments, maybe going back centuries?
This was my thought too… King Charles’ new cipher is in the news recently (“CR”), and I assume that as long as ciphers and such have been a thing, they have also been embroidered on the ceremonial robes.
I guess too the issue is “how?”. Silk-screening is a tedious process suitable for mass production, and requires a dye or such that won’t bleed. Sewing cut-out letters on does not easily lend itself to mass production in the days before automation, nor does embroidery.
But those wall hangings with embroidered sayings and bible verses were a thing in the 1800’s and earlier. The Bayeux tapestry was embroidered with plenty of words describing the action, in the 1100’s, so the concept of text embroidery on cloth was obvious and practiced back then.
I can do you some knitted socks with the word “Allah” from around the 1100s…
The twelfth-century coronation robe of the Holy Roman Emperors has a Kufic text embroidered round its edge.
Some years ago, I saw a little girl, maybe 8 - 10 yrs old, wearing a pair of pants with somewhat stylized lettering on the seat. I am somewhat ocd about text so I focused on it trying to make out what it said. I soon realized that I was staring at a little girl’s butt with the word juicy written on it. Something like this, only on a little girl.
Who on earth thought that was a good idea?
Oh, they make diaper covers for babies with words you wouldn’t believe.
I don’t know who thought of it. But over-sexualized clothing for little girls has been around awhile.
The Greeks have been doing embroidery since forever. (China too, I’m sure)
There were women and probably men who were very adroit with needlework. And faster than you’d think. Also they had slave labor to choose from.
That’s really gross.
Juicy is a brand of clothing for preteens and teens. They are very sexy styled. I find them* reprehensible to be marketing to young girls.
They’re not the only ones.
*And their parents who purchase them.