car chalk
n.— «Though several brands, colors and sizes currently are available, what is called “glass chalk,” “car chalk” or “car graffiti” began as liquid shoe polish. The process of scraping the shoe polish off car windows with razor blades must have lost its appeal, for now this new product has been made specifically for writing on windows with less mess.» —“Unleash your inner graffiti artist” by Kim Eiten News Tribune (La Salle, Illinois) Nov. 2, 2006. (source: Double-Tongued Dictionary)