The term means a fear of empty spaces. I know this feeling well. I frequently feel compelled to fill my canvases. Periodically, I will paint out large areas to give the picture plane places to rest.
A note about my thumbs. They may appear disproportionate. I have always related to Sissy Hankshaw (Even Cowgirls Get The Blues) as I have very large thumbs. I have convincingly explained to the curious that I was born without them and doctors attached my big toes to my hands.