Try, make an attempt, especially if you don't think you have much chance, but nothing to lose.
Come and have a go if you think you're hard enough.- Originally this was a chant by British football hooligans, meaning 'come and fight if you think you are strong enough', but it has passed into general use as a way of challenging someone.
Re: Understanding these sayings: Have a shot at, Have a crack at, have a go at...
How about this one righ here:
Sean, you know what the odds are against winning the lottery?
What? Four to One?
About 7,000,000 to one.
I still have a shot.
Also other one here:
No, I liked it. And I still like it. If people I don't even know look at me and want to fuck me, it means I really have a shot at being a model. Which is great, because there's nothing worse in life than being ordinary.
In this two cases, do they all mean having a real chance or possibility of doing it?