Their relationship 'went stale'. [=became dull or predictable]
She felt 'stale' [='bored'] in her job.
a room filled with 'stale' smoke
(..) I wanted italian. It's interesting. - Life. It's all about new horizons. It keeps you from getting stale. Exactly. You know, these last couple of weeks I've been taking stock of a lot of things. (..)2009 TvShow: Heroes Title: Chapter One 'Orientation/Jump, Push, Fall' Season: 4 Episode 1