This post from alt.usage.english demonstrates that most dictionaries are rarely, if ever, made completely from scratch:
“Here’s the whole genealogy: The ‘Macquarie Dictionary’ is an expansion of the ‘Hamlyn Encyclopedic World Dictionary,’ which was based on Clarence Barnhart’s ‘American College Dictionary.’ This connects to information I contributed to Wikipedia: The ‘American College Dictionary’ was a stripped down revision of the ‘New Century Dictionary,’ which was an abridgement of the ‘Century Dictionary and Cyclopedia,’ which was an expansion of the ‘Imperial Dictionary,’ which was an expansion of Noah Webster’s ‘American Dictionary.’”
