Borg Control Language, as others have said, Google's configuration language for "Borg", the precursor of Kubernetes. Both Jsonnet and Cue are descendants of BCL, developed by people who were previously involved in BCL development.
Jsonnet is a direct descendant of BCL, "BCL done right". Cue is an indirect descendant of BCL, you might say "BCL rethought".
Imagine Jsonnet, but internal to Google, old and quirky. It's not public, but the name has been leaked for a while now.
"Old and quirky" is an understatement. It's a god-awful mess, with semantics so murky that people end up just copying patterns and hoping that it'll do what they want it to do. And it can't be killed, because every bit of those implementation-defined semantics are used by someone.