Someone who catches fish using a gill net, or a person who works with gills in fish processing, or in dialect, someone associated with water streams.
From 'gill' (a net for fishing, from Old Norse 'gili'). The '-er' suffix creates the agent noun meaning 'one who does.' The term appears in medieval English fishing documents and regulations. It's also used in Scottish dialect for stream workers.
Medieval England had so many specialized fishing words—'giller,' 'netter,' 'fishmonger'—that you could tell someone's entire occupation from a single word, a pattern reflected today in words like 'programmer' or 'florist.'
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