Created at Mathcounts 1996, a mathematics competition for junior high school students.
"Onomatopoeia" refers to a word like "boom" or "cuckoo" that sounds like what it means. The word comes from "onoma-" for "name", and "poi-" for "make". This inversion has 180 degree rotational symmetry: turn it upside down and it looks exactly the same.
I created this inversion on the spot in response to a challenge from the audience at a talk I gave. Other long words I have improvised as inversions during talks include humuhumunukunukuapuaa (the state fish of Hawaii), and supercalifragilisticexpialidocious (the longest word you ever heard, from the movie Mary Poppins).
This particular inversion works rather easily: the round O's and A's coincide nicely. The extra loop on the first O is optional, but necessary on the third O, to keep it from reading as a U.