Waifu
Original Meaning
"Waifu" (A phonetic version of how the Japanese might pronounce "Wife") originally was used to denote an anime character - specifically one's favoured Anime character. It could be said to be the personification of an ideal significant other
On /tg/
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 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. 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.
In Short
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.