/ˌded ˈweɪt/ the heaviness of a person or object that cannot or does not move by itself: She may be small but, when I have to carry her upstairs after she's fallen asleep, she's a dead weight (US also she is dead weight). SMART Vocabulary: related words and phrases. Measurements of weight & mass.

Why do we say dead weight?

When we use "deadweight" or "dead weight" in modern English, we mean that an idea or person is a complete burden. The term comes from the maritime industry. The term is a noun and this author is using it as an adjective. It's not the best use of this word that I've seen in my life.

Is it deadweight or dead weight?

or deadweight

the heavy, unrelieved weight of anything inert: The dead weight of the bear's body was over 300 pounds. a heavy or oppressive burden or responsibility. the weight of a railroad car, truck, etc., as distinct from its load or contents.