The idea was, “on fire” == “has legs” == “hot”
Could just as easily be changed.
Other candidates might be:
“check it out”
“look it up”
Or left out entirely.
Another possibility is:
(repeated, or tiled, with the underscores replaced by spaces; hard to implement on a t-shirt, though)
The possibilities are nearly endless!
Maybe a contest should be held?
How about a free one-year subscription to wordorigins.org?