Metaphorical, Literal
Definition #1 — at least at the end(?) of a long, snowy Minnesota winter — is easy: “Watch your back!” literally means, “Be careful you don’t strain your back [while shoveling].”
The comment is usually offered solicitously, often by a loved one; it’s sometimes preceded by “better” or “be sure to” (“You’re going to shovel out the driveway? Fine. But, better watch your back . . .”).
Which leaves two, more metaphorical definitions.
Definition #2: Nefarious, often involving skullduggery (synonym: “beware”); said to a friend or ally, sometimes sotto voce (in a hushed tone):
“I don’t trust the new Marketing VP — watch your back . . .”
Definition #3: Tactical.
Sergeant: “My platoon is attacking the enemy sniper from the left flank.
Commanding Officer: “OK, but watch your back.”
See also, “Which “Done” Did They Mean?“; “The Many Guises of “Hot Dog’“; “The Multiple Meanings of “What Do You Know?!?”“; and ““Shut Up!” Has 9 Different Meanings? Shut Up!! (No, YOU Shut Up!).”
Plus: ““Take it Outside”: the ORIGINAL Meaning“; and “Small Word, LOTS of Meanings: “On.”
