People on public transport who are sleepy or lazy often rest their heads against the window, "head smear" is the blurry vaseline like substance left on the glass that comes from the oil and grease in their hair.
"Would ya look at that view! It's beautiful!"
"Guh, makes me wanna vom. Some dirty chump has left head smear all over this side, and I'm not wiping it down."
"Heya honey, what? ... I'm not sure what station I'm at because there's head smear everywhere and I can't see the platform."