Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for unsuitable; he lists it under incongruous.
Where he writes about it:
Incongruous applies to relations, unsuitable to purpose or use; two colors are incongruous which can not be agreeably associated; either may be unsuitable for a person, a room, or an occasion. — from incongruous