The phrase originates from 19th-century England, where "kettle of fish" referred to a cooking method where fish was boiled in a kettle at social gatherings. Saying "that's a different kettle of fish" meant something was distinct or separate, similar to how different fish would be in separate kettles.
Playground, meadow, plaza.
Aside from the Latin, the most common term for lying on your back is supine.
bairn
skelp
glaur
Orthopnea is the medical term meaning shortness of breath when lying flat.
No, it's French.
bubbly jack
teuchter
The term is collinear.
philanthrope