One version is pom <- pomegranate <- pomegran't <- immigrant. Back in the 20s, which I'm told is first mention of the term, immigrants were nearly all from the UK.

They had historical good reason to be resentful of the British state. Up above, I've been posting odd snippets from old newspapers I found on- line. Steal a lamb? 15 years transportation, a large family left destitute (they probably were before). 12 y-o girl and 8-y-o boy, 15 years each for setting fire to a haystack. The one thing I haven't seen transportation for (yet) is poaching.