An archaic (now dialect) word for ‘spider’ from Old English attorcoppa: attor, -poison, and perhaps cop, head or copp, cup. and was also once used as an insult to refer to a miserable and bad-mannered person.

Along with 'Tomnoddy', ‘lazy lob’ and ‘crazy cob’, Bilbo used the word attercop to torment the Mirkwood Spiders and get them to chase him, allowing him to sneak back, while made invisble by his magic Ring of Power and free his dwarf companions, all of whom had been captured by the spiders and hung upside down in cocoons of sticky webs.
Encyclopedia entry originally written by PotbellyHairyfoot