Looking for the origin and definition of the slang term "shave-tailed
lieutenant".  Probably military in origin.

Hello ursidae and thank you for your question.

The word "shavetail" refers to a Second Lieutenant who is not very
experienced in Army matters.  The saying comes from the practice in
the army of shaving the tails of newly broken pack mules to
distinguish them from seasoned ones.

See:

"Main Entry: shaveˇtail 
Pronunciation: 'shAv-"tAl
Function: noun
Etymology: from the practice of shaving the tails of newly broken
mules to distinguish them from seasoned ones
1 : a pack mule especially when newly broken in
2 usually disparaging : SECOND LIEUTENANT" 
http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=shavetail

Also see:
"A ?shavetail,? Essin explains, was a new initiate, a rookie unproven
in a pack train and designated by a roached mane and cropped tail,
whereas the ?bell sharp? was an experienced mule who knew where to
line up and could identify her saddle."
http://www.etsu.edu/news/20000085.htm

Thank you for your question, and if you need any clarification of my
answer, do not hesitate to ask prior to rating my answer.

Very best regards

THX1138

Search strategy included:
shavetail mule
://www.google.com/search?hl=en&lr=&safe=off&as_qdr=all&q=shavetail++mule

Good information.  :) I have heard the term used disparagingly, but
never knew its origin.