"1th" is what people who don't know any better use in place of "1st". "2th" and "3th" also may be used by the same individuals.
I ranked 1th place in this race! Then again, I only raced against 3 other people...
1st base, only really, REALLY depressing. Often involves use of nose or teeth, and results in never seeing each other again.
"Dude, I just got to 1st base!"
"More like 1th base, bro! Did you even brush you teeth?"
national Greg day, celebrate everyone named Greg!
larry: oh hey Greg *hugs* its national Greg day!, January 1th
Greg: thank you