Idiom: third wheel
Note: A third wheel could be an unwanted person in a group, such as someone's mother going together with her son and his date to the movies. However, they do not have to be unwanted. For example, your best friend might go to a movie with you and your spouse.
You want the person there but they feel awkward because they don't have a romantic partner with them. They feel like a "third wheel."
Many things that move have two or four wheels.
A bicycle has two and a car has four. A third wheel can make thing unbalanced (unless it's a tricycle). So the idiom really refers to an awkward situation or someone who shouldn't or doesn't want to be the third person in a group.
— Is there any way we can go out alone tonight? I'm tired of your little brother always being the third wheel.
— I really hate being the third wheel but it's better than staying alone at home every weekend.
— I feel like a third wheel when I go to the movies with my sister and her boyfriend.
— Since my husband passed away, the only time I go out is if I'm the third wheel with my girlfriends and their husbands.
— I'd rather be the third wheel than not go to the dance.
— My dad sent my little sister out with us tonight as a third wheel so we won't do anything that isn't rated G.
— Last night, I didn't want to go out with my brother and his wife and be a third wheel but I'm glad I went along because I met a really nice girl at the bar.
— Finally, I'm dating someone so I'm no longer the third wheel when I go out with other couples.
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