1. birdlime. 2. a : a caustic highly infusible solid that consists of calcium oxide often together with magnesium oxide, that is obtained by calcining forms of calcium carbonate (as shells or limestone), and that is used in building (as in mortar and plaster) and in agriculture —called also quicklime b : a dry white powder consisting essentially of calcium hydroxide that is made by treating quicklime with water c : calcium.