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.
Best Answer
"That's a mercy" means approximately "Well, that at least is something to be grateful for"; it expresses satisfaction or, more often, relief upon perceiving some redeeming positive feature in an otherwise distressing situation. I'd guess that it originated in pious circles—we may presume that the "mercy" involved is God's mercy in sparing us a worse outcome—but it long since lost any explicit religious reference.
It is not an exclusively American expression; Joost Kiefte gives you an example from Shaw, and Google will show you examples from Wodehouse and Dickens. It's quite old-fashioned now, but not yet defunct. Here's a recent example from a public radio station in upstate New York:
I can see no reason for its use in the passage you give. Like Robusto, I'm pretty sure the passage was written by someone with an imperfect understanding of English.