Where does "Santa" in Santa Claus come from?

The name "Santa Claus" comes from a dialect of Dutch, where the word was "Sante Klaas".

In this case, it was not a feminine suffix; the word evolved into Santa, which only coincidentally looks like the feminine form of saint in some languages.

(The Dutch word does come from the same origins as the Spanish and Portuguese, incidentally; most Germanic languages borrowed this Romance word and made it their own.)

Sanctus Nicolaus:

The modern figure of Santa Claus is derived from the Dutch figure of Sinterklaas, whose name is a dialectal pronunciation of Saint Nicholas, the historical Greek bishop.