Guys, What Do You Mean When You Say This?

When I was stepping off the bus, a guy who was waiting to get on looked at another girl and then me and went, "Oh shit!" My thought is it meant he thought I looked good. Silly question, I know but I'm curious to hear from a guy's perspective. Guys, when you say this, what's the reason?

Updates
5 mo
Just to clarify he said it like, "Ohhhh Shiiiiiiit"
Updates
5 mo
And I'm actually 23
Guys, What Do You Mean When You Say This?
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