crustyoudaoicibaDictYouDict[crust 词源字典]
crust: [14] Latin crusta meant ‘hard outer covering, shell’ (it is related to a number of words, including ultimately crystal, denoting a hard surface caused by freezing). Old French acquired it as crouste (the modern French form croûte formed the basis of croûton, borrowed into English in the early 19th century), and passed it on to Middle English as cruste. Crusta formed the basis of the modern Latin adjective crustāceus ‘having a shell’, applied in the early 19th century to the crustacea or crustaceans. And a custard was originally a kind of pie enclosed in a crust.
=> croûton, crystal, custard[crust etymology, crust origin, 英语词源]
lobsteryoudaoicibaDictYouDict
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