One more entry:

The American Thesaurus of Slang (1942) gives us "terrier" under 771: (Railroad) 10: Section hand.

In other places, "terrier and shamrock" is offered as slang for "corned beef and cabbage."

One of my other dictionaries (an English publication) suggests "terrier" comes from "Territorial Army" and describes the derivation as being used "punningly."

All of these tell us how the word was used, which we already knew, but not why. And so it goes.

Sandy