that ain't no lie
A very informal way of emphasizing that something that was just said is the truth. A: "We're gonna get in some serious trouble if we get caught." B: "That ain't no lie—so let's not get caught!" All this legal stuff at work has me really stressed out, and that ain't no lie.
That ain't no lie.
Rur. What was said is true. (The highly informal word ain't is built into the expression.) Tom: Sure is hot today. Jane: That ain't no lie. I'm plumb exhausted, and that ain't no lie.