"If you have a waifu, you have no laifu."
"Waifu" (a phonetic version of how the Japanese might pronounce "Wife") originally was used to denote a female anime character - specifically one's favored anime character. It could be said to be the personification of an ideal significant other, but most often denotes a character one felt attraction to beyond the average anime girl. Originated as part of a one-off gag in Azumanga Daioh! for whoever even cares.
Despite being exactly the kind of weeaboo identifier that 4chan attacks like hungry wolves, it's also the kind of short-and-snappy universal term that faggots can't help but use anyway when they stop giving a shit about looking cooler than they are.
A less common variation is "Husbando," the male equivalent of waifu. Never mind the fact that adding -o at the end makes it sound more Spanish than Japanese. Even less common is "Daughteru," used to describe a strictly platonic and paternal attraction for a younger anime girl as a way to indicate a repressed desire to become a responsible father and to have a family to care for (sexual attraction to an underage-looking anime girl is a different issue altogether). And then there's the weirdos at /k/, who like to describe their favorite gun as their "Raifu." We'd make an overcompensation joke, but that would make more sense than what having an infatuation for firearms would imply.
On /tg/ any female character is often referred to as Waifu, particularly if she is a character made on /tg/. Occasionally it is used seriously to express love for (or the desire to have sex with) a female character. More often, it is used negatively, as a slur - implying that said character is only popular because she is female and /tg/ wants to bone her, or to suggest that the person being addressed has an unhealthy obsession with said character. Some have gone so far to use waifu to mark entire entities or groups, such as the Sisters of Battle. Such usage of waifu is most often accompanied by much rage.
This new usage has come about as the direct result of meme decay, specifically the meme leaving forums like 4chan and finding its way into broad use by the anime community as a whole. The term became, like the much older desu, so overused by immature and irritating 12-year-olds that /tg/ has summarily executed any worth it had left.
Waifu is a term used to disparage any female OC on /tg/ regardless of quality. If you want to see hilarious rage, keep making more.
An Alternate Take
It's important to note that, according to some people, "Waifu" is more a verb or adjuctive than a noun: It refers to the "relationship" between the weeaboo and the character, regardless of the intent of the author.
Thus, unless a character stands accused of being designed to inspire Waifuing, no character can be a Waifu in-and-of themselves, because any character can be Waifued or Husbandoed. Yes, including Nurgle and Cthulhu. Under this view, the appropriate name for a character designed to be Waifu'd is "Waifu-bait".
Those Who Stand Accused of Being Waifus
(Or, in the alternate, "Designed to be waifus" or "Waifu-bait")
- Brown Elves
- Elesh Norn
- Sisters of Battle
- The Primarchs' Daughters
- Just about any female Mary Sue written by a male author.
- Any other female character who is not utterly repulsive (and even then there's a chance they'll be accused anyway)