Why doesn't "knee long" make sense?

[D Huong]

English Language Tests, Intermediate level

ESL/EFL Test #103 "Charlie's Knees", question 9

After they were married, Charlie had to work very hard and he was always knee ......... in decorating and painting the house.

(a) deep
(b) level
(c) steep
(d) long

English Language Tests, Intermediate level

ESL/EFL Test #103 "Charlie's Knees", answer 9

After they were married, Charlie had to work very hard and he was always knee deep in decorating and painting the house.

Correct answer: (a) deep

Your answer was: incorrect
After they were married, Charlie had to work very hard and he was always knee long in decorating and painting the house.
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Why doesn't "knee long" make sense? Thanks in advance.

D Huong

[Mister Micawber]

.
One's knees are a measure of height, not length. The metaphor comes from the physical image of standing in something-- water, mud, trouble-- to the height of one's knees, so that the water or (idiomatically) the trouble is 'knee-deep'.

You can also be waist-deep or up to your neck in difficulties, projects or preschoolers, for instance.
.
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Native English teacher at Mister Micawber's