Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for unmanageable; he lists it under rebellious.
Where he writes about it:
Ungovernable applies to that which successfully defies authority and power; unmanageable to that which resists the utmost exercise of skill or of skill and power combined; rebellious, to that which is defiant of authority, whether successfully or unsuccessfully; seditious, to that which partakes of or tends to excite a rebellious spirit, seditious suggesting more of covert plan, scheming, or conspiracy, rebellious more of overt act or open violence. — from rebellious
While the unmanageable or ungovernable defies control, the rebellious or seditious may be forced to submission; as, the man has an ungovernable temper; the horses became unmanageable; he tamed his rebellious spirit. — from rebellious