Pineapple hair is a common form of updo for those with long, frizzy or braided hair. It is, essentially, a ponytail done at the top of the head, but done by someone who's hair doesn't 'tail' so much as... poof. Exuberantly.
The image evoked is that the person's head is the pineapple, and the hair is the big bunch of spiky leaves on top. Traditionally, this was done as a way to put hair up for the night, and so was tied up with a scarf. As it became more popular as a somewhat bohemian fashion statement, more effort was put into keeping it up without the scarf, although both forms are common enough today.
As this is a somewhat informal term, it is used in somewhat informal ways. There are FAQs on-line explaining "how to pineapple your hair", and others explaining about the "pineapple hair technique". It is embraced as a verb (pineappling one's hair), a noun (do a pineapple), and an adjective (the titular 'pineapple hair'). Regardless, it is delightfully descriptive, and perennially popular, both as a hairstyle and as a turn of phrase.
HairQuest 2025