I've tried very hard to think of what this may be. I've come up with two similar gestures. See if one of these fits your context. 1. In my part of the world, people who are cowardly, or afraid may be described as being "chicken." This happens most often with little boys. So if one boy wants another to do something and he refuses, others may try to pressure him by asking him if he is chicken. To illustrate, and to make the pressure greater, they may tuck their thumbs into their armpits, flap, and maybe even walk with a bit of a bounce to immitate a chicken in dramatizing their taunt. In this context, it is part of a taunt. 2. The second one came from the culture of a few decades ago. "Hillbillies" or country people were often humorously portrayed as dressed in clothes that didn't fit (because they were poor), and usually as being enthusiastic, but uneducated and "backwards." Suspenders were often worn by many classes of people to hold ill-fitting pants in place, but with the hillbilly humor in particular, the suspenders were made a humorous item to illustrate the ill-fitting clothes. In this type of humor, when one person was proud of himself, he might be portrayed as putting his fingers under his suspenders with his thumbs at about the front of his armpits. He would puff his chest out and maybe even rock back and forth with a big, confident smile on his face. I haven't seen any reference to the second one in years, but I think little boys still illustrate a chicken when playing like the first example. I hope someone else can think of another example :).

"Uncoordination"? Mystery to me as well... I've looked around and the consensus is that it's probably made up. Maybe someone else has news...

I have never heard of it.