Penny  n. (pl. For separate coins -ies, for a sum of money pence) 1 british coin and monetary unit equal to one-hundredth of a pound. 2 hist. British bronze coin and monetary unit equal to one-two-hundred-and-fortieth of a pound.  in for a penny, in for a pound exhortation to total commitment to an undertaking. Pennies from heaven unexpected benefits. The penny drops colloq. One understands at last. Penny wise and pound foolish mean in small expenditures but wasteful of large amounts. A pretty penny a large sum of money. Two a penny easily obtained and so almost worthless. [old english]