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Talking in one's sleep, usually in a nonsensical manner. It's possible to have a multiple-sentence "conversation" with someone who is "ferdering". The ferderer will not remember the conversation.

Etymology: Some English-speaking people who talk in their sleep say words that sound like "ferder ferder ferder".
"I just heard Joe talking. Is he awake?"

"Nope, he's just ferdering."

by Captain Leroy February 12, 2009