Meaning of “Wasshoi(わっしょい)”

“Wasshoi(わっしょい)” is a usual phrase we use at Japanese festival.
I don’t know the meaning of it, but with this word, we can match the rhythm with others and feel getting power.

If you watch the Japanese festival, you can hear this word.
While carrying a Mikoshi, the carriers yell “Wasshoi wasshoi”.
*Mikoshi is a kind of small house of gods.