Matey
Disclaimer: I don't own Endeavour.
Strange had gotten into the habit of calling people "matey" a long time ago.
The first time he called someone that was, when he met an 8 year old, crying boy. The boy was all alone and 16 year old Jim Strange couldn't just ignore him. So he had sat down in front of him and talked to him. The boy had been inconsolable, but gradually Strange managed to calm him down. Turns out he had gotten himself lost. It took a while, but he found out the address. After that he had brought the boy home.
During all that, somehow the word "matey" had slipped out. It seemed to help, so Strange stuck with it. It seemed to be friendly, without being patronizing. It made people feel understood in their struggles and sometimes that was all they needed.
That was why Strange called all those that were sad, shocked or borderline hysteric "matey".
When Strange first met Morse, the DC was neither of those things. Rather he was being a clever clogs and a bit rude.
Still, there was something in those big blue eyes and his awkward posture that told Strange, this man needed a friend.
A part of the man looked lost, just like the little boy a decade ago had.
It had felt naturally to call him "matey". And that moment Strange decided to look out for Morse.
The end
I hope you liked this short insight. Please leave a review.