When one eats peanut butter the smell of peanut butter sticks with them for a extended period of time even though all evidence of the peanut butter is gone.
Often related to other items of similar consistancy. (read:feces)
A dog does his business outside and comes in to sit beside you. The dog smells very much like his dirty business even though his anus is completely free from 'cling-ons'. This is the peanut butter effect.
One is doing laundry and notes the 'peanut butter effect' on a pair of underwear. The underwear may only inhibit a foul smell or may be accompanied by 'skid-marks' after being well used.