The Google NGram viewer can show you how often a word has been used in books.
If we restrict to the 20th century, and add a smoothing on 10, what result do you get for the year 2000 for your word?
My current lowest is
footling - just 0.0000025%!
What's the most unpopular word you know?
(Update: Words have to have an entry in the google list to count.)
Leader board:
0.0000000101% -
Infandous - drjd
0.0000000156% -
philomathy - drjd
0.00000015% -
haulageway - cromag
0.0000003% -
ecdysiast - pdurrant