O.E.
loppestre, corruption of L.
locusta "lobster, locust," by influence of O.E.
loppe "spider," a variant of
lobbe. 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." But OED says the L. word originally meant "lobster or some similar crustacean, the application to the locust being suggested by the resemblance in shape."
Locusta in sense "lobster" also borrowed in Fr. (
langouste), Old Cornish (
legast). The ending of O.E.
loppestre is the fem. agent noun suffix (cf.
Baxter, Webster; see
-ster), which approximated the L. sound. Slang for "a British soldier" since 1643, 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]
[lob-ster] 
) -ster, (especially referring to two or more kinds or species