Perilous comes from Old French perillos, from Latin periculosus ('dangerous'), from periculum ('danger, trial'). The form is best read as a historical transfer or inheritance, with its older language material carrying the main semantic load. As an adjective, it carries the sense "full of danger or risk", with the older formation still helping explain the modern use. Relevant historical forms include Latin periculosus and Latin periculum. Derivative forms include perilously, perilousness; related words include peril, imperil.