It comes from quechua Wasa : Back . And of the expression mandrapa, which is to shake like rag. I mean, Wasamandrappa would be kind of like coming in the back. Because many of them arrived in Lima, the word lost the initial meaning, so it began to refer to the pinga as slang. And this one spread by the obscene it sounds.
* Only one "like" per meaning and day, the more "likes" the meaning will appear higher in the list
