To Multiply
To Increase
intransitive verb, ichidan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means multiply and the verb vocab version means to multiply or to increase.
What is multiplying? Your use of the word "fu" (ふ) for when you laugh. You used to say just "ha ha ha" like everyone else. Now the occurrences of you saying fufufu is really multiplying. You might also notice that this is similar to the word 増える, which means "to increase." This has a similar meaning and the same reading, use that to your advantage!
なんだか毛が殖えてるなあと思ったら、植毛手術をしたんだぁ！なるほどねぇ。
I thought your hair had multiplied somehow, but you had hair replacement surgery, huh? I see.