what does "good lord" exactly mean in this condition?
In TBBT S01E04, Sheldon said:" Fine. We’re travelling forward at, good Lord, 51 miles an hour. Now let’s assume that your brakes are new and the callipers are aligned, still, by the time we come to a stop, we’ll be occupying the same space as that Buick in front of us, an impossibility that nature will quickly resolve into death, mutilation and… oh look, they built a new put-put course. "What's more, What dose the "put-put course" mean?

Question

what does "good lord" exactly mean in this condition?
In TBBT S01E04, Sheldon said:" Fine. We’re travelling forward at, good Lord, 51 miles an hour. Now let’s assume that your brakes are new and the callipers are aligned, still, by the time we come to a stop, we’ll be occupying the same space as that Buick in front of us, an impossibility that nature will quickly resolve into death, mutilation and… oh look, they built a new put-put course. "What's more, What dose the "put-put course" mean?

Answer

It is like saying OMG.  When he was about to tell us the speed, the speed surprised him.  He was shocked by his own information.

Answer

"good lord" is expressing astonishment or amazement in this context. like he cannot believe they are traveling at this speed. similar to "oh my god!"

"putt putt course" is related to mini golf. :)