I do not know the precise etymology, but Indraja is a fresh spring spirit in the Lithuanian pantheon. The name is presumably related to the Sanskrit ind, which in Sanskrit means to drop, or droplets, as in indu, the moon, or indra the rain god; similar words for water bodies etc. abound in this region. She is also the planet Jupiter, and was to the bride of the thunder god. Thursday, or Thor's day, is presided over by Jupiter in a lot of Indo-European cultures.