I came across the phrase "That's a mercy!" in a textbook dialogue. To put it in context I've reproduced the whole dialogue as below:
Bobbie: You look like hell, dad. What's on?
Sam: Nothing special. You snored a lot last night, and I thought it was a thunder so I closed the windows one by one. Then you began grinding your teeth from midnight.
Bobbie: And you thought a mouse was coming into the kitchen and watched out the whole night?
Sam: That's a mercy!
The whole dialogue makes sense as some sort of family teasing. It's just Sam's reply in the end "That's a mercy" which stands out a bit for me. What does this mean? Is this some kind of quick repartee? In what context do we normally use it?
The textbook has a slant of American English, to put in context.