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2.
From Joss Whedon's (unfortunately) short-lived television show "Firefly."

Purplebellies are members of the Anglo-Sino Alliance, so named because of the purple-tinted armor they wore during the Unification War.

A strongly derogatory term.
Did you hear what that purplebelly called Serenity?
Gorram purplebellies can't take the sky from me.
by Chris the Browncoat September 26, 2007
 
1.
An Alliance soldier or officer. (Used in reference to an Alliance Captain in Firefly Episode "Bushwhacked".) The Alliance's official colors include purple and are used on the abdomen (belly) of Alliance troop body armor.
"But Captain, did you hear what that purple belly called Serenity?" — Kaylee Frye
by —thoughtstorms— March 26, 2009