What TikTok 'underconsumption core' trend means for your money: It's 'romanticizing being middle class,' content creator says

Using only one water bottle. Finishing that tube of makeup before buying another. Owning furniture that's been passed down through generations.

This isn't the lifestyle that social media influencers promoting their Amazon storefront or their brand discount codes show. So-called "underconsumption core," however, is one of the latest personal finance trends to go viral on TikTok, with many videos about the topic receiving millions of views.

On social media, the "core" ending is often used to describe a shared aesthetic among users. Non-personal finance examples include cottage core and goblin core. Underconsumption core showcases the old items that people are still using.

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The trend is coming into play at a point when consumers feel increasingly cash-strapped.

"I think it's romanticizing being middle class," real estate agent Sophie Hinn of Okoboji, Iowa, told CNBC.

She has posted about how she embodies the trend, including using old towels for cleaning rags and filling her home with furniture that's "thrifted, gifted, repurposed, [or] family hand-me-downs."

In a July video — one of the first using the term "underconsumption core" — 18-year-old Maya "Liu" Feldman from Germany shows the old hair dryer she still uses, and clothes from seventh grade and holey jeans she still wears.

Feldman's TikTok immediately went viral, racking up more than 436,500 likes and 2.3 million views. Many users commented that they lead a similar lifestyle or that they were motivated to spend less after watching Feldman's video.

"I didn't really have a lot as a kid, and it kind of gave me this