Could someone please clarify this kanji?  I can't find the character in the dictionary that I have.  Does it mean 'not' or does it mean 'knot'.  I've read Heisig's blurb but I can't seem to make any sense of it even though he says the primitive meaning is 'knot', but then the last sentence throws me off. 

This is what I get, we're using 'knot' to remember the abstract keyword 'not' right?  but 'KNOT' is the actual meaning... right?  (so why doesn't he just use KNOT as the keyword?). .. -_-!