This is what Carl Zeiss in Germany is writing on their website about the lens name "Sonnar":
"A lens design with relatively few glass-to-air surfaces, invented by Dr. Ludwig Bertele at Carl Zeiss in 1930 to provide the fastest lenses of that day for 35 mm photography offering speeds up to f/1.5 and well controlled veiling glare.
This is where the name comes from, containig the German word for sun, 'Sonne', the symbol of utmost brightness.
Since the Planar® lens with its many glass-to-air surfaces could successfully take over the 'fastest lens' role from the Sonnar® lens, after Dr. Alexander Smakula at Carl Zeiss invented anti-reflex coating in 1935, the Sonnar® design turned out its other virtues.
Today it is the basis for compact high-performance medium telephoto lenses with speeds up to f/2.8 (in case of the Sonnar® T* 2,8/150 FE lens), very elaborate correction of lens errors (in the case of the Sonnar® T* 5,6/250 CFi lens and the extreme case of the Sonnar® Superachromat 5,6/250 CFi lens), incorporating expensive optical glass types, and offering very even corner-to-corner illumination. "