thack /θæk/exclamation 1noun 2verb 2forms 3 exclamation1 •The sound of a thack.Whishhh—THACK! The strap is a blur as it comes to meet my outstretched hand. — 2012 noun2 •The weatherproof outer layer of a roof, often specifically thatch. •A stroke; a thwack. verb2 •To cover a roof with thack. •(dialect) To strike or thump (someone or something); to thwack. formsthacked · thacking · thacks