Prawn as a noun is an object, like a video or picture, that has to do with sex. As a verb, "prawning" is the act of pleasuring one's self while watching prawn.
An undesirable ex-girlfriend, ex-shag-buddy, ex-one-nite-stand, ex-partner. prawns cause an uncertain uneasiness with the new spouse even though there is no need as the ex has been discarded for reasons such as lack of spine, thick skinned harshness, lack of personal hygiene, creepy arms or legs ... ETC.