An American Southern peasant expression used to prod a workmate to complete a task in a timely manner. It was first used outside a farm field in the 1934 King Vidor Depression-era back-to-nature epic "Our Daily Bread" in the mouth of one of the field workers on an irrigation project relaying the order to let the reservoir water through the irrigation ditch in the last scene of the film.
It also has obvious sexual connotations, whether referring to the job of leading a bull to mount a cow or a man's marital duty.
The expression was made extremely popular by US country/blue-collar comedian Larry The Cable Guy (circa 2003) as the epitome of everything down home vulgar and redneck.
First worker: Let the water through!
Second worker: Go, go, go!
Third worker: Let'er rip!
Fourth worker: Do it now!
Fifth worker: Get'er done!