it is a word created from the pronunciation of a slurring of words together
Perhaps the same could be said of a number of very
formal words such as
whomsoever,
whereunto and
heretofore, words found in legal documents but seldom heard in 'real speech'!
Also, to describe the formation of such hallowed terms as 'slurring together' might not be altogether accurate.

Indeed, words such as
gimme and
gonna are now nearly all classed as 'rare' in Chi, although I don't think that was always the case!
A suggestion -
whatsit should have the same classification as
whodunnit, which I believe is spelt with only one 'n' (
whodunit) in N.America, and which is not only misspelt but also grammatically inaccurate!
I'm willing to bet that most Chi solvers know the word, however!
it does make me think of Mrs Whatsit, a character in the classic book A Wrinkle in Time[/i
It's a long time since I read this very fine sci-fi / fantasy novel, but, if I remember correctly, Mrs Whatsit was the name of one of three witches (?)/weird sisters (?) / goddesses (?), the others being Mrs Who and Mrs Which.
That
might suggest that Ms L'Engle considered
whatsit as valid a word as the other two names?