They're worn with one's arms inside (8)
I believe the answer is:
holsters
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for holsters that I've seen before include "Sheathes for a handguns" , "Sheaths for guns" , "Gun holders" , "Receptacles for hand--guns, pistols etc" , "Sheaths for handguns" .)