"Umpstable" is a word to describe somebody.
By calling somebody "umpstable" you're saying that they are showing off, but that are attempting to make it look natural. Therefore, they don't come under the "show-off" category, instead they come under the "umpstable" category.
In transformers when Sam is showing Mikaela the light and disco ball, stretching his arm back indicating where the light reflects- showing his muscles. This everyone, is being umpstable!