It's an icon with a paint bucket about halfway tipped over. There is liquid running out of the bucket because it is tipped partially over. There is a slash line running through the middle of the paint bucket starting at the top left through the bottom right.
This No Filling icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 5,700 icons, all in the same style and quality.