Definition
An accidental messy hairdo resulting from the wearing of a helmet.
How People Actually Use It
Messy hair. The flat, lifeless appearance of hair, or wild, 'uncontrollable' appearance of hair when a motorcycle helmet is removed. Most affected are women and it's a common problem for those with a coiffure.
"When she took off her helmet, everyone could see she had 'helmet hair'. Her 'helmet hair' took a lot of brushing-out before her hair looked nice again."
Source: Urban Dictionary