1. To have impaired vision, often due to drunkenness. I once got so drunk I couldn't see straight. I'll never do that again.

2. To not be able to function, typically due to anger, frustration, fatigue, etc. When I found out that my brother took my car without asking, I was so angry with him I couldn't see straight. I'm so tired right now that I can't see straight. You'd better drive me home.