What does root word fen mean?

The root word "fen" actually doesn't have a specific meaning on its own. It's likely you're thinking of the word "fen" which is a noun meaning "low, wet, marshy ground".

Here's a breakdown:

* Fen: A type of wetland characterized by:

* Low-lying, flat terrain

* Standing or slow-moving water

* Growth of grasses, reeds, and sedges

There isn't a root word "fen" that's used independently to form other words.