A fart made while sitting in a chair. A chair has four legs. The person has two arms and two legs- that's eight legs- thus the spider reference. With time the term has been misused so often, it has now come to mean any fart.
( Person sitting next to you lifts one leg and farts ). I have to go, that Barking Spider just put me off my lunch.
A higher pitched fart, typically resulting when the fart is either forced, or interestingly, when the fart is attempted to be squelched. Should never be confused with a barking tree frog, the tone of which is clear and resounding.
background: if a spider could make a sound it would "bark" and it wouldn't be as loud as a dog bark, and a fart isn't as loud as a dog and it's near to the ground. If someone farts and everyone looks around to see who done it,but everyone acts innocent one
might say... It was a Barking Spider