Literally, "I will
go in to the
Altar of
God".
The beginning of a traditional Roman Catholic Mass in Latin. The proper response from the altar server is, "Ad Deum, qui lætificat juventum meam". ("To God, who gives joy to my youth.")
Also, among certain noders, and you know who you are, the beginning of a bong hit for Jesus.
JudyT says: "For many of us, I'd imagine, this is much better known as the first words spoken by stately, plump Buck Mulligan at the beginning of Ulysses."
SOURCE: http://nine.optera.net/~gfvonb/trandem.com/Catholic/Manual/Mass/serving.html