What's the meaning of "to be dead on one's back?

[Hury]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #557 "English Slang Idioms (195)", question 2

Janis hoped she would be able to drive home safely. She normally had to cope with sleep deprivation but this situation was worse than usual. She worked three back-to-back shifts and the work had been physically demanding as well. She had been required to lift several patients onto gurneys and, then, had to wheel several of them long distances. She was dead on her ......... and was afraid that she did not possess the requisite reaction time one needed to safely drive.

(a) back
(b) head
(c) feet
(d) pillow

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #557 "English Slang Idioms (195)", answer 2

Janis hoped she would be able to drive home safely. She normally had to cope with sleep deprivation but this situation was worse than usual. She worked three back-to-back shifts and the work had been physically demanding as well. She had been required to lift several patients onto gurneys and, then, had to wheel several of them long distances. She was dead on her feet and was afraid that she did not possess the requisite reaction time one needed to safely drive.

Correct answer: (c) feet

Your answer was: incorrect
Janis hoped she would be able to drive home safely. She normally had to cope with sleep deprivation but this situation was worse than usual. She worked three back-to-back shifts and the work had been physically demanding as well. She had been required to lift several patients onto gurneys and, then, had to wheel several of them long distances. She was dead on her back and was afraid that she did not possess the requisite reaction time one needed to safely drive.
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What's the meaning of "to be dead on one's back? Thank you for your kind attention

[Alan]

Hi,

This is an expression meaning exhausted/extremely tired. There is a related expression: 'dead tired.'

Alan
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