English Phrase: As thick as thieves
Do you have a terrific friend? Perhaps this person is your best friend. If you are very close to this person and you can share your intimate thoughts or worries with them, then you are as thick as thieves.
Thick has the meaning of being close together. Often, when we see a bamboo grove or forest, the bamboos are very close to one another. In other words, the bamboos are very thick.
Thieves usually do bad things together, so they need to trust each other. Thieves need to believe that their friends will not tell the police what they are doing.
I think that if we think about it in this way, it is easy to understand that the phrase as thick as thieves means that two people are very close friends.
