It means 'BFF' in Japanese. It's a slang which is kind of old school, and short word for 'ZUTTO (always, forever) TOMODACHI (friend). It got popular for teenagers. They often take a 'PURIKURA' which is the shortened form of the 'PRINT CLUB'. It's a photo booth that has been very popular among young girls, best friends, and young couples. People can take a picture by themselves, and it comes out in sticker forms. They can write or draw any thing on the photo with the pen function like, their name, date, or message, and make the photo stickers fun and original! So, 'ZUTTOMO' as 'BFF', a lot of 'best friends' people write it on PURIKURA. I'm not sure that people are still using this word lately, but most of the time, it's only used by teenagers.
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