The phrase is a combination of the word’s “win” and “incest”, which creates a meaning very different from the ones.
Instead of the original meaning of the word “incest”, which is a sexual relationship (most of the time abusive) with family members, “wincest” refers to the exact opposite, where both sides agree to intercourse.
Origin of the term
It is not possible to define the exact origin of the phrase, as there is no date of it first being used set in stone.
The very first mention to find on the internet is an entry on the website called Urban Dictionary, where a user has submitted the meaning and explanation of the phrase in 2006, however it is possible that the use of it on the messaging boards 4chan and reddit predate the entry.
Spread of the term
Given the meaning of the phrase and the internet’s obsession to share porn with the world, it has spread like wildfire on image sharing websites and message boards like 4chan and Reddit.
You can find users creating threads for the phrase multiple times a day, with content that you would expect from the meaning.