The term is actually "Shaved Tail"
Originally it designated a mule in the army that had just been broken in, and the idea was everyone would know to be careful of that mule while his tail hair was growing back.
By the time it grew back, it would be a seasoned veteran and therefore reliable.
As SPEMack618 referenced, they often were used to designate a "butter bar" officer, an Ensign or 2nd Lieutenant (their collar devices are brass, hence, butter bar).
Butter bars and shaved tails were especially useful in the Navy when working on the gyroscopic compasses, and you needed a tube of relative bearing grease. Trouble is, . . . they just seemed to never come up with it.
May God bless,
Dwight
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