that does it

Also, that does the trick. The last requirement has been fulfilled; that accomplishes it. For example, That does it; we're ready to send in the application now, or That last screw does the trick—it's fully assembled. Grose's Dictionary of the Vulgar Tongue (1823) defines do the trick as “accomplish a robbery or other business successfully,” and presumably the it in the first term stands for the trick. [Early 1800s] Also see that's that.