(n) Used to reference a dog's butt hole immediately prior to the dog having a bowel movement. Right before a dog has a bowel movement, their butt hole starts to expand and pucker, causing an uncanny resemblance to Donald Trump's mouth. The Trumpy Butt is a signal to the owner or dog walker that the dog is about to go. The state following Trumpy Butt is often referred to as "Brown Eye."
Rover's been looking for a poop spot forever, but now he has Trumpy Butt so I know he's about to go.