lobsteryoudaoicibaDictYouDict[lobster 词源字典]
lobster: [OE] The Latin word locusta denoted both the voracious grasshopper, the ‘locust’, and the ‘lobster’ or similar crustaceans, such as the crayfish (if, as has been suggested, the word is related to Greek lēkan ‘jump’, then presumably the ‘grasshopper’ sense was primary, and the ‘lobster’ application arose from some supposed resemblance between the two creatures).

English has borrowed the Latin word twice. Most recently it came in the easily recognizable guise locust [13], but lobster too goes back to the same source. The radical change of form may be due to the influence of the Old English word loppe ‘spider’ – the Old English precursor of lobster was loppestre or lopystre.
=> locust[lobster etymology, lobster origin, 英语词源]
lobster (n.)youdaoicibaDictYouDict
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]