To do something hammer-and-tongs is to work very hard at something, and to do it for quite a long time.
Example : "God. My damn roommate and his girlfriend were going at it hammer-and-tongs again, and I haven't been able to get in my room for over an hour!"

It's meant to evoke the image of blacksmiths shaping metal, constantly pounding on it, never stopping until it is finished.

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