There's this phrase: Skeletons in the Closet. (or 'Skeleton in the Cupboard' elsewhere).
As I understand it, it refers to something you'd rather keep secret.
In our playing, we all have things we do better or worse. The natural human tendency is to avoid pain and, in music, this often means avoiding those things we don't do very well.
For trombonists, this often means avoiding such unpleasantries as extreme soft playing, singing, or awkward intervals. For some, it means avoiding lip slurs or long tones.
Those little weaknesses don't usually get better through avoidance.
Often they begin to loom large in our minds!
Embrace them! Make them the centerpiece of your practicing.
In other words:
Don't just practice what you can already do well.
Practice what you don't do well.
Embrace those skeletons!
