I just used the phrase "just giving you a heads-up" for the first time in years, and it got me thinking about the origins of the expression and variations of it.
Heads-up (nominal) is essentially the equivalent of advance warning and heads up (phrasal verb in the imperative) is basically pay attention. Some examples:
The boss is going to come by later, so I just wanted to give you a heads-up!
Heads up, the boss is going to come by later!
There's more discussion in this Q/A, but the origin of heads up is not my question. I want to know about variations of the phrase (in the sense of advance warning), and their origins.
I know of a few variations off the top of my head:
look/watch out: Watch out, the boss is coming by later and he's not happy.
look alive: Look alive, the boss is coming by later for an inspection.
but I don't know where these expressions came from. What are some variations of heads up in the sense of advance warning and the origins of those variations?