What is the context?
It doesn't say whether the hands are up or down, straight or bent.
I would say: hands straight down at your sides (as in a pose for an exercise).
It is a complete sentence. It is just said as an instruction, as a part of a set of instructions which I listened to. I interpreted that both hands should be put straight down, the usual position of hands when we stand and do nothing.
It is a complete sentence. It is just said as an instruction, as a part of a set of instructions which I listened to. I interpreted that both hands should be put straight down, the usual position of hands when we stand and do nothing.
As part of a series of instructions, Hands at your side! is an imperative sentence. An implied verb and possessive pronoun are omitted, "put your". Although it might seem logical to say sides, only the singular is used.
