I am not sure if this old saying is an Australian thing, more likely, it probably originates in the UK but it sounds very Australian. My understanding is it is an insult, meaning your not that smart but I like bricks they can be very useful! You can even build a whole house with them and a double brick house is even better. Quieter, cooler, stronger, as homes go it does not get much better.
Two bricks are strong but I suppose it is pretty hard to teach bricks new things. They are pretty stuck in there ways! Primary school was my first real exposure to Intelligence. I thought I was ok but being graded and marked on academic tests clearly showed I was only average at best, maybe thick as one brick. lol. But like a brick I was more than useful in many other ways.
I am not sure the schooling systems measurement of intelligence is totally the best guide. Many times I struggle to pass and get good great grades but end up much closer to the top when it actually comes to getting stuff done. Like what is the point of being measured as super intelligent if you cant get things done.
This all reminds me of work site inductions tests. Over the years I must have done many hundreds. Amongst our crew I was usually the slowest to finish until I work out just to read the pick a box answers first before reading the question, if a all.
That makes me think that Intelligence should be more measured in being able to adapt and adjust rather than just a photograph memory and quick recall.