Ah, so the eternal question: What is a waifu? A waifu, considered as a best girl from a non-action live animation, or one with qualities you fancy for your significant other is more than what you keep at heart.
Shigaraki is true to his word.
My research shows that it began around 2007.
The canonical origin for this term in its current usage is Azumanga Daioh, a 2002 anime.