comes from the old-fashioned method of washing clothes in a wash tub. the clothes were pounded and swirled around with the poss stick.
lad: "look at her, would you give her one?"
fella: "aye lad, i'd bend 'er ovver 'n' give her the possin' of her life"
mate1: "what would you give her?"
mate2: "aye she's bonny enough, seven out of ten"
mate1: "hey, what about this one then?"
mate2: "oh aye, a reet good possing is what i'd give her"