I used brew cask list and noticed that one cask (infinit) had an exclamation mark next to it: (!)
Does this have any particular meaning? --help did not make me any wiser :(

It means the cask no longer exists. You installed it when it did. Search for it now, and it won鈥檛 find it.
Nice terminal color scheme @thibmaek ! get where? :)
@michahell It's Toy Chest :)
How can a cask with a (!) be uninstalled at this point?
@fpedroza use brew cask uninstall --force.
That doesn't work for me, miccal.
$ brew cask uninstall --force send-to-kindle
==> Running uninstall process for send-to-kindle; your password may be necessary
==> Removing launchctl service com.amazon.sendtokindle.launcher
==> Removing files from pkgutil Bill-of-Materials
==> chmod: Unable to change file mode on /usr/libexec/cups: Operation not permitted
Error: Command failed to execute!
==> Failed command:
/usr/bin/sudo -E -- /bin/chmod -- 777 #<Pathname:/usr/libexec/cups>
==> Standard Output of failed command:
==> Standard Error of failed command:
chmod: Unable to change file mode on /usr/libexec/cups: Operation not permitted
==> Exit status of failed command:
#<Process::Status: pid 7390 exit 1>
@karstenwolniak that seems like it is more of an issue with the cask than with cask though.
This error was why send-to-kindle was removed - you will have to remove it manually.
Most helpful comment
It means the cask no longer exists. You installed it when it did. Search for it now, and it won鈥檛 find it.