One of my guy friends called me his waifu?

I don't know I keep thinking about this lately because I've never been called someone's waifu. If you're not familiar with the term, please Google it--it's a common anime term. But regardless, the other day my guy friends and I were reunited with an old friend we haven't seen in awhile. Unbeknownst to some of the guys, they didn't know I knew the friend. The guys were like "oh, okay." And my old friend smiled and said "Yeah (my name) is my waifu." I blushed, but took it as a lighthearted joke (we've just been friends). What's your interpretation? Why would he call me that?
One of my guy friends called me his waifu?
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