The either flat, static-y, greasy, thin, or just downright disgusting hair that results when you forgot your shampoo/conditioner at home and are forced to use the terrible crap that hotels call 'complimentary shampoo'.
"OH NO! I forgot my shampoo! I have to use this no-name crap! I wanted to look good tonight, but now I'm going to have Hotel Hair."
"Did you forget your shampoo?"
"Yeah, how could you tell?"
"You've got horrible Hotel Hair."