1
general::
Run away, as in When they heard the sirens, the boys turned tail. This term alludes to an animal's turning its back in flight. [Mid-1500s]
American Heritage Idioms
2
general::
Phrase(s): turn tail (and run)
Fig. to flee; to run away in fright. • I couldn’t just turn tail and run, but I wasn’t going to fight that monster, either. • Sometimes turning tail is the only sensible thing to do.
McGrawhill's American Idioms And Phrasal Verbs