original meaning of the idiom "as thick as thieves"

[Carla Louse]

English Idioms and Expressions, Advanced Level

ESL/EFL Test #15 "Teaching your Family to Drive", question 8

They've settled their argument at long last and now they're as thick as thieves.

(a) very clever
(b) very friendly
(c) very careful
(d) very cautious

English Idioms and Expressions, Advanced Level

ESL/EFL Test #15 "Teaching your Family to Drive", answer 8

They've settled their argument at long last and now they're very friendly.

Correct answer: (b) very friendly
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Hi,

What is the original meaning of this idiom?

Thanks
Carla Louse

[Mister Micawber]

.
thick as thieves

Intimate, closely allied. This term uses thick in the sense of "intimate," a usage that is obsolete except in this simile. [Early 1800s] (American Heritage)

Besides the fact that criminal gangs often have to live and work closely together, the idiom alliterates.
.
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Native English teacher at Mister Micawber's