What does boto stand for

Boto derives its name from the Portuguese name given to types of dolphins native to the Amazon River.

SO, I don't think it stands for anything or it has full form. Though this could only be best answered by the Team of AWS, who has come up with this name, but documentation suggests, what I have written at the start.

Its just catchy name and has some link the Amazon and the Amazon River

From internal Wiki:

The Boto was named after it because it swims in the Amazon, and all the good river and city names were already taken.