get your rear in gear

[English Learner]

English Language Tests, Intermediate level

ESL/EFL Test #479 "English Slang Idioms (221)", question 1

"You agreed to start work at 7 o'clock and, here you are, asleep at 8 o'clock. It is time to get your rear in ......... and get out in the yard. You might still finish by the end of the day but you will have to work hard," Steve's dad told him.

(a) inspection
(b) gear
(c) passing
(d) walking

English Language Tests, Intermediate level

ESL/EFL Test #479 "English Slang Idioms (221)", answer 1

"You agreed to start work at 7 o'clock and, here you are, asleep at 8 o'clock. It is time to get your rear in gear and get out in the yard. You might still finish by the end of the day but you will have to work hard," Steve's dad told him.

Correct answer: (b) gear
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please, could you explain this answer?

[Beeesneees]

'get your rear in gear' - a slang idiom which means 'get yourself sorted out / get started.'
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"Inside every old person is a young person wondering what happened."
Terry Pratchett