In salsa it stems from the Afro-Cuban religion called Santería. You hear it (and other santería phrases) a lot in Cuban salsa. Most often you hear the full phrase “Aguanile mai mai” and it is part of the toque for the saint (orisha) Oggun.
The term Aguanile mai mai is praising the Yoruba orisha called Ogun. Ogun is the god of war and metal. He is the diety that works day and night without ever resting. He is the primordial black smith. He held the secrets of metals and forging tools. He held these secrets to himself until the orisha Oshun (river diety) seduced him out of the forest so that civilization could be created.
He is also praised in Cuba in the religion of Santeria (La Regla de Ocha Lukumi). The language used in Santeria is called Lukumi which is an offshoot of the Yoruba language of Nigeria West Africa.
Aguan= a cleansing. (also refers to a basket that is used in ceremonies where different types of foods and animal offerings are contained after the inititates have cleansed themselves with these items.
Note: this is not proper Yoruba spelling. This is Lukumi where there is really not a standardized spelling and words may vary slightly depending on the priestly lineage of the Santeria practitioner.
Contrary to popular belief, mai mai does not mean “water”. The word for water in the Lukumi language is “omi”