That is a lame excuse in some contexts. However, if Superman leaps over an entire building, then is asked to jump off a curb and declines, I think you can safe

That is a lame excuse in some contexts. However, if Superman leaps over an entire building, then is asked to jump off a curb and declines, I think you can safely assume he could do it, but doesn't feel like it. If I jump off a curb and say I could jump over that yonder building (if I felt like it, but I don't). Then, yes, that would be lame.