I “invented” the term stomache the other day, by noticing how the words stomach and ache overlap. Why should you say “stomach ache” when they both already share an ach? Take that ach out! Here is a visual depiction of the future evolution of the phrase “stomach ache”:

Urban Dictionary has a definition of its own:

1. Stomache

An idiot who can’t spell “stomach”.
Guy1: Man, my stomache is killing me.
Guy2: It’s stomach, dumbass.

2. stomache

Upset stomach, contracted form of “stomach ache”
You can’t sit on my lap, pet, I have a stomache.

The rest of the Google references will take you to pages about gastroenterology, under the assumption that stomache is really just a common mispelling of stomach. Unfortunately, I’m writing this to tell you that it’s not. It’s its own new portmanteau word, replacing “stomach ache.”