Shavetail

In the US Army a “shavetail” is a new officer, usually a 2nd Lieutenant, who is fresh out of OCS (Officer Candidate School). Their distinctive short haircuts set them a prat from the more experienced officers. The term has come to be used to mean anyone who is inexperienced with their jobs.

It goes back to the days when the Army used pack animals, such as mules, to haul supplies. New animals would require some breaking in to be of much use, one that was accomplished the handlers would shave their tails. This let everyone know that they were new and set them apart from their more seasoned compatriots.

Basically, calling some a shavetail means use caution as they are new on the job.