Hair lacky

A hair tie or hair elastic. The term 'hair lacky' is an amalgamation of the words 'hair tie' and 'lacky band'. Lacky band being a shortened version of elastic band or rubber band.

"Hey, do you have a spare hair lacky? I've got P.E. next and I don't want my hair getting all messy."
"Nah, sorry mate. I've got a lacky band but their super painful to use instead of a hair lacky."