Mental

Derives from the adjective temperamental: a person used to sudden changes in mood.
Because Americans can't spell, the word has been shortened to the last two syllables, but everybody has forgotten about the original spelling. That's why shortening words is always a bad idea.

"Hey, that guy down there is so temperamental."
"Is he what? Speak English, man."
"I said he is TEMPERAMENTAL."
"Oh yeah, he is mental all right!"

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Jay walking

The trajectory of a pedestrian illegally crossing a large thoroughfare against incoming traffic.

When you cross a street with lots of incoming traffic, you will walk parallel to the flow of traffic and in a direction opposite to it. Then, when you see an opening, you will turn and dash through the other side anticipating (hopefully) the incoming vehichles. Seen from above, your trajectory takes the shape of the letter J, hence the expression "Jay walking."

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