To tousle something is to dishevel it—that is, to make it untidy or unkempt. Tousle is usually, though not always, used specifically when a person’s hair is being so treated.

// Vic stood in front of the mirror and tousled his hair, trying to get a cool, disheveled look.

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“One of her hands tousled her long hair, which she wore down, and the other hand hovered in front of her skirt as she hooked a thumb in its waistband. She paired the fashion set with a choker necklace and dangling hoop earrings.” — Meghan Roos, Parade, 21 Mar. 2024

The verb tousle today is typically used for the action of mussing someone’s hair playfully (“tousling the toddler’s hair”) or fussily (“tousling her tresses for that just-woke-up look”), but the word’s history is a bit edgier. Tousle and its synonym touse come from -tousen (“to pull or handle roughly”), a frequentative of the Middle English verb touselen. (A frequentative indicates repeated or recurrent action; sniffle, for example, is a frequentative of sniff.) Both tousle and touse have older meanings having to do with rough handling in general; before hair was tousled, people were—ouch. It’s no coincidence that another frequentative of -tousen, the Scots word tussillen, is the ancestor of the English verb tussle, meaning “to scuffle” or “to fight or struggle with someone by grabbing or pushing.”