Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for unstained; he lists it under pure.
Where he writes about it:
In moral and religious use pure is a strong word, denoting positive excellence of a high order; one is innocent who knows nothing of evil, and has experienced no touch of temptation; one is pure who, with knowledge of evil and exposure to temptation, keeps heart and soul unstained. — from pure