Describes something of poor quality, often related to sound or taste, giving a negative impression. That speaker is really tinny; I think it's broken.
Pro Tip 2/3
Sound Quality
Used when high-pitched or sharp sounds lack richness or depth, typically in audio equipment or music. Avoid those headphones, they make everything sound tinny.
Pro Tip 3/3
Not for Quality
Steer clear of using 'tinny' to describe high-quality products; it's usually for cheaper items. This pan is a bit tinny; my old one cooked much better.