That's rusty.
[Slang] THAT'S RUSTY. - Brush Up on Describing Skills in English
Skills have declined.
When you hear "that's rusty," it indicates that someone’s skills or knowledge are not as sharp as they once were due to lack of practice. It’s like rust building on metal; without regular usage, abilities can deteriorate.
Example Sentences
His tennis skills have gotten rusty after not playing for years.
His tennis skills have deteriorated after not playing for years.
You can tell his Spanish is a bit rusty, but he's improving.
You can tell his Spanish has become less fluent, but he is getting better.
Ever since I stopped practicing, my guitar playing has become really rusty.
Since I stopped practicing, my guitar playing skills have worsened.