watch one's back

verb2

1. •
   To remain wary of being attacked from behind by surprise; to remain watchful to prevent a predictable attack from being a surprise.

   • the kind of dive bar where you better watch your back
2. •
   (figurative) To be wary of betrayal or an otherwise unexpected attack.

   • Gotta watch my back now that you turned me around Got me walking backwards into my hometown — 2011