lobster (n.) Look up lobster at Dictionary.com
marine shellfish, Old English loppestre "lobster, locust," corruption of Latin locusta, lucusta "lobster, locust," by influence of Old English loppe "spider," a variant of lobbe. The ending of Old English loppestre is the fem. agent noun suffix (as in Baxter, Webster; see -ster), which approximated the Latin sound.

Perhaps a transferred use of the Latin word; trilobite fossils in Worcestershire limestone quarries were known colloquially as locusts, which seems to be the generic word for "unidentified arthropod," as apple is for "foreign fruit." OED says the Latin word originally meant "lobster or some similar crustacean, the application to the locust being suggested by the resemblance in shape." Locusta in the sense "lobster" also appears in French (langouste now "crawfish, crayfish," but in Old French "lobster" and "locust;" a 13c. psalter has God giving over the crops of Egypt to the langoustes) and Old Cornish (legast). As slang for "a British soldier" since 1640s, originally in reference to the jointed armor of the Roundhead cuirassiers, later (1660) to the red coat.
Sir William Waller having received from London [in June 1643] a fresh regiment of five hundred horse, under the command of sir Arthur Haslerigge, which were so prodigiously armed that they were called by the other side the regiment of lobsters, because of their bright iron shells with which they were covered, being perfect curasseers. [Clarendon, "History of the Rebellion," 1647]