Expressions: Is It

I should be posting more entries on this topic…because it’s fascinating to me. Right now the expression that I’m obsessed about here is: “Is it.” Here’s how it’s used…I say to someone…Andrew likes to work hard. The response: Is it. Or…I say, my cat loves to catch lizards. The response is still: Is it. It seems to be used like we would use…”oh really.” It seems like it has to be short for some larger clause or sentence…but I’m just not sure.