To be at daggers drawn is to be in a state of open hostility. Daggers or knives were routinely carried as protection from very early times. Therefore, it is not surprising that drawn daggers literally meant open hostility. The figurative use of the expression, however, dates from the late 18th/early 19th century. To look daggers at somone, meaning to glare at somone with open hostility dates from the early 1600s.