A correspondent writes to ask about the utterance 'She was very thick with the gardener', encountered in the Hercule Poirot episode, The Halloween Party. 'I looked it up in a dictionary and it seems it's an old-fashioned way of saying "she was very friendly with him". Then I saw another expression, thick as thieves (is it still used?) .What I can't really understand is how/why the word thick (usu. 'not thin'; 'stupid') came to be used in the sense of 'friendly'.

The original sense, in Anglo-Saxon times, is of material extension, where it's regularly opposed to thin, as in a thick wall. Then it came to be applied to density and abundance (thick hedge, thick hair, thick mist...) and size (thickset) and extended to thickness in general (where it's not opposed to thin), as in 'the cover is an inch thick'.

It then generated several figurative senses meaning 'excessive in some disagreeable way', especially in the phrase a bit thick ('indecent' or 'indelicate'). It's the sense of 'density of contact' that led to the figurative sense of being 'close in association' - in other words, being 'intimate' or 'familiar'. It seems to have been a late 18th-century development. And quite a few similes have since emerged, such as as thick as glue, as thick as peas in a shell, and as thick as thieves, some of which have become proverbial.

The idioms are still used a lot, but the use of the single word to mean 'intimate' is certainly old-fashioned now, redolent of P G Wodehouse and Evelyn Waugh.