na-adjective
entirely useless; completely worthless
Emphatic reduplication of 駄目 (no good), meaning hopelessly bad or beyond repair. Often used in casual, colloquial contexts.
See also: 駄目
このパソコン、もうダメダメだよ。
This computer is completely useless now.
彼の運転はダメダメで、怖くて乗れない。
His driving is totally hopeless; I'm too scared to ride with him.
interjection
no no!; don't!
Used as an interjection to strongly forbid or reject something, often in a playful or emphatic tone. Equivalent to repeating 駄目 for emphasis.
See also: 駄目
ダメダメ、そんなことしちゃ！
No, no, don't do that!
ダメダメ、まだ食べちゃダメ！
No, no, you can't eat it yet!
駄目 is the base word meaning 'no good' or 'not allowed'. ダメダメ is an emphatic reduplication, stronger and more colloquial.
Reduplication of 駄目 (だめ, 'no good') for emphasis. The exact origin of 駄目 is uncertain, but it is often associated with the game of Go, where 駄目 refers to a useless move.