"Lame" is generally something disappointing. A lamer is a "downer" or somebody who bums people out,or makes them feel dejected or disappointed. "You are so lame!" is an expression you use to denote mild disappoinment in a collegue. Depending on intonation, it can be very mild or actually quite serious. Say for example, a friend says " I hurt my knee I can't play basketball today." In that case you could say " You are so lame! We'll play again next week." Following with the latter sentence means that disappointed,and he's not meant to take this mild insult too seriously. On the other hand if the second sentence was "And I seriously think you're a piss weak individual!" and was spoken in anger,you probably would damage your friendship a bit

This is slang. "Lame" literally means that a person can't walk. However, when you tell a person he or she is "lame," you are saying that they are doing something that you think is not worth the time or is a dumb activity. There are many situations when you could say it. If you want your friend to go out with you on the weekend, and he tells you that he is going to stay in and paint model airplanes with his mom, you might say, "You're lame." However, I personally wouldn't say "you're." I would say, "That's lame." This way, you are talking about the activity being stupid, and not the person himself. It's a little less offensive. Either way, you should only use the phrase to joke around with a person who you are good friends with to avoid hurting feelings.