It's no secret that millennials are in debt. Massive debt. The average American aged 25 to 36 years old is in debt to the tune of about $42,000, and most of it isn't even from student loans — it's from credit card spending. Credit card debt in America surpassed $1 trillion in 2017, according to the Federal Reserve. And with living costs continuing to climb, the debt hole deepens.

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How many times have you complained to a friend — jokingly or not — about being broke? That's probably most young people reading this article. Yet statistics show that millennials are still spending money on the things that experts say would constitute huge savings if they were avoided — things that Shark Tank personality and O'Shares ETFs chairman Kevin O'Leary says are utterly, totally stupid. Here's O'Leary's advice to millennials about the top spending habits to quit, stat.

1. $4 Coffee

"$4 coffee. Incredibly stupid," O'Leary told CNBC while at the World Government Summit in Dubai this week. "Look, I know I'm going to get hate mail from all the coffee brands, but coffee costs 18 cents to make yourself. Until you have savings and have paid off your college debt, do not buy a $4 coffee, I forbid you." Personal investing app Acorns found in 2018 that the average American spends about $1,100 per year on coffee, or about $92 a month — which it says is about 30 percent more than a third of Americans spend on investing.

The price of a cup of coffee varies wildly across the U.S. and the world. A 12-ounce cup of joe from Starbucks in the U.S. costs on average $2.75, but New York City is the most expensive location for the chain, charging $3.25 for a tall cappuccino. And if you want the works, with more expensive types of milk, flavoring or seasonal specials, it can set you back more than $5. Starbucks in Europe, the Middle East and South Asia cost far more, often averaging $6-8 a cup or more.

2. Shoes

"On both sides of the equation, shoes," O'Leary says emphatically. "You don't need more than four pairs of shoes. You need flip flops, something to work out in, and two pairs of dress shoes — everything else, you're an idiot if you're buying more shoes. Because you'll never wear them, and they'll be sitting there for years." In this area, women appear to be the worst offenders — shoe retailer DSW found last year that 75 percent of women in the U.S. own more than 20 pairs of shoes, while the average man reportedly owns 12 pairs. Credit card comparison app CreditDonkey reports that the average person buys 7.8 pairs of shoes per year. And a report by Psychology Today found that, indeed, of all the pairs of shoes owned, most people regularly wear only three to four.

3. Jeans

Nobody should own more than three pairs of jeans, O'Leary believes. "If you have more than three pairs of jeans — one black, one white and one jean original — you're an idiot," he says. "You have too many, you don't need that." Today, a pair of jeans comes in at about $60 on average, "which means Americans spend about $38.5 million on denim pants every day, or $14 billion annually," according to personal finance website howstuffworks.com. And women's jeans cost an average 10 percent more than men's, says credit card comparison app CreditDonkey. On clothes in general, women 16 and over spend 76 percent more than men of the same age, the app found. The average adult aged 25-34 spends $161 per month on clothing.

Potentially massive savings