The Collaborative International Dictionary of English v.0.48:

lobsterback \lobsterback\ n. (American History)
   A British soldier in the 1700's; -- so-called because of his
   red coat, esp. during the American Revolutionary War.

   Syn: redcoat.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

lobsterback
    n 1: British soldier; so-called because of his red coat
         (especially during the American Revolution) [syn:
         redcoat, lobsterback]