To mess up something simple that you do right all the time. This usually occurs in a big situation, such as a test, interview or superbowl.
I had three tests last week and I would have had A's but I welkered parts of all of them.
To make an amazing catch (football). This catch can lead to many things either a superbowl victory, a converted 3rd or even 4th down, and possibly the game winning touchdown. This catch can only be achieved by a very elite catche. Or this may also mean you got fined for a hat.
That was a welkered catch!
I was welkered by the NFL.