Sudo apt-mark hold

Does this mean that no updates will overwrite my thunar installation?

  sudo apt-mark hold thunar
  thunar set on hold.

That is correct. Thunar will not be updated

See my comprehensive analysis on AskUbuntu:

The short answer - use apt pinning instead:

So my conclusion is the following: apt-mark is usable only if you install/remove/upgrade software only with apt-get, otherwise you should use pinning (i.e. /etc/apt/preferences.d/), it is more reliable and straightforward.

Thanks for the info.

But this is confusing as to the format of the file needed. And I want to use geany as my editor.

 Usually on my Ubuntu 16.04 LTS, I lock package versions with APT-pin functionality.

    For example if I want to pin `meld` to version *1.5.3-1ubuntu1* I create the following pin-file:


   cat <<EOF | sudo tee /etc/apt/preferences.d/pin-meld
  Package: meld
  Pin: version 1.5.3-1ubuntu1
 Pin-Priority: 1337
 EOF

I use such code snippet for automation - I do it from simple text terminal.
In your case you should determine current thunar version with apt-cache policy thunar (for example you got x.y.z-MubuntuN from it) and then fill /etc/apt/preferences.d/pin-thunar with info below:

Package: thunar
Pin: version x.y.z-MubuntuN
Pin-Priority: 1337

But if Thunar's version is greater than in your current distribution, then it would not upgrade until new Ubuntu version released.

I made a file called pin-thunar

 Package: thunar
 Pin: version 1.8.2-1~18.04
 Pin-Priority: 1337