Meaning of “Usomatsu(嘘松)”

“Usomatsu(嘘松)” means “(almost definitely) a lie” or “(almost definitely) a lier”.
This is an internet slang from about 5 years ago.
“Uso(嘘)” means lie, “matsu(松)” is from “Osomatsu-san(おそ松さん)” which is a title of anime.

What is “osomatsu-san”?

“Osomatsu-san(おそ松さん)” is a kind of remake anime of “Osomatsu-kun(おそ松くん)” which is written in 1962 by the most greatest manga artist.
It’s a comedy anime of Sextuplets.
Their names are “osomatsu(おそ松)” , “karamatsu(カラ松)”, “choromatsu(チョロ松)”, “ichimatsu(一松)”, “jushimatsu(十四松), “todomatsu(トド松)”.
They are sextuplets, so they looks same but their personality are quite different.
Same faces but different personality. People can find their own oshi-matsu(推し松).
This anime generated a lot of big fans.

Why uso-matsu?

As you know, on Twitter, some people lie to get attention.
One day, a woman tweeted a story. She said “I met a guy who resembles osomatsu-san” and wrote manga of the story and posted the manga on Twitter.
The story she says looks definitely lie.
People think it’s a lie.
However she keep insisting that it’s a true story and he is real.
Still people think it’s a lie and someone start call him “kaku-matsu(架空松)”. Kaku(架空) means imaginary.
In the end, someone start call him “usomatsu(嘘松)”.

This is an origin of “usomatsu(嘘松)”, maybe.
Now, the meaning of it is different from first version. It means just “lie” or “lier”.

I should’ve wrote this earlier. I didn’t think this slang takes root such a long time. I still see this slang on internet.
By the way, opposite slang of this is “hontoumatsu(本当松)”.

Example Conversation

On twitter
A: Guess what! I’m moved, bla bla bla…
On an online bulletin board
B: Look this tweet.
C: It might be usomatsu
C: Umm… I think, surprisingly hontoumatsu!

On twitter
A: 聞いて、感動的なの。なんたらかんたら〜
On an online bulletin board
B: 見てこのツイート
C: どうせ嘘松だろ
D: いや、これは意外と本当松

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