To pay attention (usually because danger is imminent). Often used as an imperative. You need to watch out for all these cords—they're a serious tripping hazard.Watch out! There's a car coming!
watch out
v.
1. To be careful or on the alert; take care: If you don't watch out, you'll fall on the ice.
2. watch out for To be careful of some danger: Watch out for falling rocks!