This simile means that someone is crazy or behaves very strangely. In the past many people who made hats went insane because they had a lot of contact with mercury.
I might do or say silly things occasionally, but in this instance I know what I am doing (Usually used when someone questions your application of common-sense).
A Johnny-come-lately is someone who has recently joined something or arrived somewhere, especially when they want to make changes that are not welcome.
Moral fibre is the inner strength to do what you believe to be right in difficult situations
Example: He lacked the moral fibre to be leader
(In American English the correct spelling is 'fiber'.)
When an older person behaves foolishly, it seems worse than when a younger person does the same, especially in relationships, as an older person should kknow better.
If you have the courage of your convictions, you are brave enough to do what you feel is right, despite any pressure for you to do something different.