What do you think "Rosebud" means in Citizen Kane? My theory is that there was a single flower that grew in the snow when he was a kid.?

It means he started to crap his pants.

It’s also slang for butthole

I think it was about his childhood and that snow toy he had whatever it's called.

It was the name of his sled.  That's the easy part of this, they actually show you that it's the sled.  The hard part of the plot is this:  if he was alone when he died, how did anyone else know what his last words were?  There was no one there to hear him say 'rosebud'.  

Flowers don't grow in snow, it was his sled.

It was the name of his sled when he was a child.  It represents the lost innocence he had as a child before becoming wealthy and successful while sacrificing the person he once was.