1840-50, Americanism; shave + tail1; orig. in reference to unbroken army mules, whose tails were shaved for identification
Examples from the web for shavetail
That's where you start, second lieutenant, shavetail.
Slang definitions & phrases for shavetail
shavetail
noun
A second lieutenant, esp a newly commissioned one
[1891+ Army; perhaps fr an early 1800s sense, ''unbroken Army mule''; perhaps also fr the fact that a newly commissioned officer might have shoulder straps made with material cut off the shirt-tail]