You know there is a Big Mind and a small mind. Sometimes the Big Mind wins over the small mind and sometimes it is the other way around.
When the small mind wins over, it is misery and when the Big Mind wins, it is joy.
Small mind promises joy and leaves your hand empty. Big Mind may bring resistance in the beginning but fills you with joy.
The word Guru means great. Jaya means victory. Deva means one who is fun-loving, playful, light. One who is playful is often not dignified and when one is dignified, he is often not playful.
Jai Guru Dev is victory to the Big Mind in you that is both dignified and playful. That is what Jai Guru Dev means: "Victory to the Greatness in you."
You do not say victory or hail to the Master as he has won over already. You say victory to your own Self, your own Mind, which is being veiled by the small mind.