What does In-a-gadda-da-vida by Iron Butterfly mean?

In the Garden of Eden.

A commonly repeated story says that the song's title was originally "In the Garden of Eden" or "In the Garden of Venus" but in the course of rehearsing and recording singer Doug Ingle slurred the words into the nonsense phrase of the title while under the influence of LSD.

Another theory is that the misspelled words are actually Bhagavad Gita, which is the name of a sacred Hindu text. https://en.wikipedia.org/wiki/In-A-Gadda-Da-Vida_%28song%29

in-a-gadda-da-vida. seems to be the drunk way of saying: "In the Garden of Eden"

Iron Butterfly frontman and keyboardist Doug Ingle wrote the song while drinking an entire gallon of Red Mountain wine. When he tried to play the song and sing the lyrics for drummer Ron Bushy, Ingle slurred his words so heavily that "In the Garden of Eden" came out as "In-A-Gadda-Da-Vida". Bushy wrote down the slurred words phonetically. When Ingle saw them later, the band agreed that the mangled phrase sounded exotic, mystical, and perfect for the era's psychedelic rock vibe, so they kept it.