I looked this name up on several portuguese sites. Some sites - they don't seem too reliable - they state the finnish meaning, and a german one, making it a version of
Hilma.
This makes sense to me, as the H is silent in Portuguese, and there are similiar versions (
Ilda -
Hilda,
Enrique -
Henrique,
Eloísa -
Heloísa). Also, the name was most popular in the south, where there is a large german community. And
Hilda has the same curve of popularity as
Ilda, only less popular.
Ilda wasn't that popular, though.