Literally, “lame” means “injured”. Example: the horse is limping because it has a lame leg. In the context of your sentence, however, “lame” means “very unimpressive”. It is not necessarily rude, but it is always negative. Be careful about using it, because it comes across as highly critical. The word is used mostly commonly to modify “excuse” — a “lame excuse” is a really bad excuse. Example: “Jim said he would come to my party but then didn’t show up. When I asked him why he didn’t come, he made up some lame excuse about having to work late.” It is also used to say that some activity is boring or worthless. Example: A: Let’s go bowling. B. Bowling is lame. Let’s go to the beach instead.