Gamp A big baggy umbrella. “Used humorously” chiefly in the UK; after a character in the lesser-known Dickens book Martin Chuzzlewit.
Gandy dancer An itenerant worker, particularly on a railroad. Possibly after a defunct manufacturer of tools, the Gandy company of Chicago.
Gapeworm Any of several kinds of nematode, a parasite that infects birds’ throats. (Imagine for yourself how it got the name.)
My my, that any given page of the dictionary might contain such a treasure trove.