Definition: To be the closest of friends, sharing each other’s secrets.
Example: Shelley and Pam were as thick as thieves, that is, until Pam ran off with Shelley’s boyfriend.
Iddy has previously explored the idiom ‘as thick as two short planks‘. In that case, thick had the meaning of being stupid. That isn’t the case here. Here, as explained in the definition, it means to be close, even conspiratorial.
This meaning of thick was common from the eighteenth century, but the rest of the phrase took longer to reach its current form. There was ‘as thick as three in a bed‘, ‘as thick as glue‘, and ‘as thick as peas in a shell‘ (which became our modern day ‘like peas in a pod‘). An especially entertaining one was ‘as thick as inkle weavers‘. What? You don’t know what an inkle weaver is? What a sheltered life you have led! Inkle is, unsurprisingly, a type of weave, and the poor souls in the Victorian weaving industries worked their looms in cramped quarters.
‘Thick as thieves’ appears to have surfaced in that same Victorian era. But why thieves to denote closeness? After all, another phrase claims there is ‘no honour among thieves‘. Thieves, when planning their next caper, would converse quietly, heads close together so as not to be overheard. It helps to think of movie versions of Victorian thieves, muttering away at a corner table in a darkened pub. They developed their own codewords and secret languages to help disguise their intentions, sometimes speaking certain words backwards, or using a forerunner of Cockney rhyming slang.
Iddy was fortunate to be down at his own local, darkened pub, when he came across these two; Thicky the master thief and his brother Nigel.