in a way that has a very small distance between two sides or two flat surfaces [≠ thickly]:
thinly sliced bread
scattered or spread over a large area, with a lot of space in between:
Sow the radish seeds thinly.
The mountain regions are more thinly populated than the lowlands.
if something is thinly disguised etc, someone is pretending it is something else, but you can easily see what it really is:
He looked at Frank's new car with thinly veiled envy.