from The Century Dictionary.
- noun The sear spring of a gun-lock.
Sorry, no etymologies found.
"No: but this little thing -- that is called the feather-spring."
At the word Fire!” continued he, “you will pull the trigger briskly, then return to the priming position, the muzzle of your firelock directly in front, the left hand just forward of the feather-spring, seize the cock with the thumb and forefinger of the right hand.”