arachnid (n.) Look up arachnid at Dictionary.com
1854, "a spider," from French arachnide (1806) or Modern Latin Arachnida (plural), the zoological name for the class of arthropods including spiders, scorpions, and mites, introduced as a class-name 1815 by French biologist Jean Baptiste Pierre Antoine de Monet de Lamarck (1744-1829), from Latinized form of Greek arakhne (fem.) "spider; spider's web," which probably is cognate with Latin aranea "spider, spider's web," from aracsna, of unknown origin.

The Latin word could be a borrowing of the Greek one, or both could be from a common root. Beekes writes, "As the word looks non-IE and since it is limited to these two languages, it is probably a borrowing." Latin aranea is the source of common words for "spider" in French (araignée, Old French araigne), Spanish (araña), Italian (aragna), etc. It also was borrowed in Old English as renge "spider;" Middle English had araine "spider" (late 14c., from Old French), which survived in dialect as arain, noted in John Ray's "Collection of English Words" (1768) as a Nottinghamshire word for "the larger kind of spiders." Also compare araneology.

Earlier noun forms were arachnidian (1828), arachnidan (1843). As adjectives, arachnidean (1853), arachnidian (1854), arachnidial (1877), arachnidal (1850), arachnidous (1833) have been used.