>>18879620It TECHNICALLY just means wife in japanese (Japanese? It's fucking taken straight from english!).
But in places like 4chan it's used to point out someone fictional you're very affectionate about.
Not an actual person you've underwent a bonding ceremony with.
FULL-ON NERD MODE: If I recall correctly, it's taken from an anime where a creepy guy has a picture of a young woman he introduces as "mai waifu".
The people around him question this, since he's a complete weirdo and by all means unattractive.
Thus the idea that a "waifu" is someone you don't have any actual relationship with that you're still very fond of. 