An open handed slap on a hot ass, occasionally followed by a hard tooshie squeeze.
The phrase has its origins in the agricultural industry, where farmers would often slap their cows hard to get their attention. This gesture is an adaptation, its primary intentions being to rouse the attention of females, and to treat them as if they were cattle.
"Damn, Jenny has a hot ass. I'm gonna give her a farm hand."
or
"Last night while I was hooking up with this girl, I have her a huge farm hand. She'll be marked for at least two weeks."
To deliver a brutal slap across a bitches face so hard that it knocks her to the floor. Similar to a pimp slap except it is usually administered with an open palm. It is also a lot harder, it requires you to place your feet firmly on the ground to avoid falling over, swivel round transferring as much momentum to the hand as possible before delivering it directly across the face.
"Joe had had just about enough of his girlfriend's bullshit, so to ease the breakup process he got a friend to drive him to her house where he threatened to farmhand her."