When you say to someone "you're so lame", you mean that they're out of touch with modern trends and are unsophisticated. The term "lame" in this context is slang for 'not being in the know'/ 'not being cool'. For example: 'She's cool, but her boyfriend is so lame.'

your are so lame mean you're so dull

It means you are not cool or you are dull.

It usually means "you are not cool" or "you are a boring person" or something similar. It's generally used a way to generally express that you dislike another person

Means being old fashioned or not cool