Femininity is the standard form of the noun referring to behavior or qualities thought to be characteristic of females. Fiminity is an old variant, and some find it appealing because it is shorter and easier to say. Yet in modern English, it appears only very rarely, and femininity prevails by an overwhelming margin.
The ngram below graphs occurrence of the two forms in a large number of English-language texts published through the 20th century. As you can see, feminity barely registers against the longer form:
It’s possible to find a few scattered recent examples of feminity:
She proved that there is enormous strength in feminity. [The Daily Sound (article now offline)]
They are a mixture of playfulness, menace and wry feminity. [Bangkok Post]
But instances such as these are far more common: