The meaning “spider” has been attested since the 1600s, but the OED conjectures that it is a back formation from “Cobweb”, with roots meaning “grab” ultimately the same as the slang word “cop” for “police officer”.

• 08.100 Lazy Lob and crazy Cob

“† cob, n.4.” OED Online. Oxford University Press, September 2015. Web. 4 October 2015.