tus·sock / ˈtəsək/ • n. 1. a small area of grass that is thicker or longer than the grass growing around it. 2. (also tussock moth) a woodland moth (family Lymantriidae) whose adults and brightly colored caterpillars both bear tufts of irritant hairs. The caterpillars can be a pest of trees, damaging fruit and stripping leaves. DERIVATIVES: tus·sock·y adj.