n (weaponry) A hand tool shaped like a small crank handle, for winding the spring of a wheel lock on a musket.
n (obsolete) A device in early steam engines for moving the valves for the alternate admission and shutting off of the steam.
n (UK) A problem, dilemma or obstacle; something unexpected or troublesome (in the phrase spanner in the works)
n (British|mildly|derogatory) A stupid or unintelligent person; one prone to making mistakes, especially in language.