About T-Shirt Design
A shirt does not need a giant explanation to be funny, and I Only Do Butt Stuff At The Gym T-Shirt proves that. The fitness humor angle gives it a little extra personality without making the design too busy.
You could wear it for birthdays, bring it out for workouts, or just keep it in the normal shirt rotation. It works well as a novelty t shirt because the joke is clear and the design has plenty of personality. If you like to buy t shirts because the saying actually matches your personality, this one makes sense. It has enough personality to be funny without looking like it is begging for attention.