One of two things.
L standing for Lose. So when your playing a game or anything really, it's possible to take a L, once again, standing for lose.
And L, as in the character from Death Note which has black hair and is majorly intelligent regarding the Kira case, of course, almost solving it.
Tim: "Darn! I can't believe I lost that game!"
Jake: "Heh, you just took a L, didn't you? "
It means taking a lost whether it being a fight or an argument
"Did you see that fight between Tyrone and Dominique?" "Yeah Tyrone took that L"
is for the way you look at me
person 1: O is for the only one i see
person 2: V is very very extraordinary
L, aka Lawliet/Ryuzaki/Hideki Ryuga. Yes a lot of names, L is from the anime/manga/Netflix movie/Death Note 1 and 2. He spends most of his time, maybe even all of it, finding out who Kira (Light Yagami), is.
Also, can someone tell Lawliet to put his carpet grippers away.
Person 1: You know, L from Death Note is kinda... emo.
Person 2: No duh, bout as emo as Elmo without the 'l'.
Person 3: L really took a l when he fell out of his chair.
L can also mean "unLucky", depending on the context it is used in.
Person 1: "My girlfriend is sick. :("
Person 2: "L"