The Village Voice runs an enlightening essay on the history of the "slogan t-shirt", which actually ends up being a history of the "slogan pin-back button" as well, since that's where wearable slogans popped up first. 

 

My first memories of slogan t-shirts date to my high school years in the mid-90s.  The store that brought them into vogue for me was Hot Topic, purveyor of all that was commercially punk and goth and symbolic of our burning rebellion against, well, we didn't know but it had something to do with being lost in suburbia.  Hot Topic took up residence at our local mall (and when I say "local" I mean 30 minutes away on the highway) and became our fashion mecca.  Yes, we could go and buy wallet chains, stickers, pins, and of course, t-shirts, all of which furthered our rebellion.  About half of the t-shirts they sold were of the band variety (Nirvana, Smashing Pumpkins, Korn, Bad Religion, Marylin Manson and tons of punk bands that actually probably sold more t-shirts than albums) but the rest were something new (to us).  A t-shirt that proclaimed "Leave Me Alone" or "whatever" or maybe even a picture of an alien with "Area 51: Believe" was the perfect way to advertise your rebellious nature.  Yes, I admit, we thought we were cool. 

 

Since that time, the industry has really exploded, I suppose the rise of cheap web-hosting and screenprinting has helped.  They're not really part of my wardrobe anymore (don't even work for casual Fridays).  I do suggest checking out Solid Threads, a local NJ company started and run by a former soccer teammate of mine, their t-shirts seem to be a tad wittier than the industry average.