Someone you are suppose to sleep with to avoid getting your ass beat. This person must get around a lot.

'Sleep with one eye open' is usually used as a metaphor in a sentence. It basically means being very cautious, careful or attentive.

Preface: This can't just be me. Ok, Def. When You have drank enough that it is inexplicably comfortable to watch TV or read with just one eye open. To open both eyes actually reduces the quality of visual perception. This stage occurs before any feeling of sickness, but well past the point of which "good judgement" is a consideration.