What does "Flat out like a lizard drinking" mean?
It means being extremely busy or working as hard as you can. The lizard comparison adds a bit of humour to the shorter phrase flat out.
The saying in a sentence
The saying
Flat out like a lizard drinking
Example
Can't chat now, I'm flat out like a lizard drinking.
When you use it
Use it when telling a mate you have a lot on, whether you are busy at work or racing to get things done at home.
When you do not
In a formal update, say what you are working on and when you can help. The saying explains that you are busy but gives no useful timing.