What is the origin of the idiom "thick as two short planks" and when to use it?

The idiom "thick as two short planks" is believed to have originated in British English and gained popularity in the 20th century. The phrase suggests that the person's cognitive abilities are so limited or dense that they can be compared to two short planks, which are inherently thick and inflexible. It is a figurative way of emphasizing someone's perceived lack of intelligence or understanding. However, it's important to use idioms like this with caution, as they can be considered offensive or derogatory when used to insult or belittle someone.