Source : Webster's Revised Unabridged Dictionary (1913)

Dangerous \Dan"ger*ous\, a. [OE., haughty, difficult, dangerous,
   fr. OF. dangereus, F. dangereux. See {Danger}.]
   1. Attended or beset with danger; full of risk; perilous;
      hazardous; unsafe.

            Our troops set forth to-morrow; stay with us; The
            ways are dangerous.                   --Shak.

            It is dangerous to assert a negative. --Macaulay.

   2. Causing danger; ready to do harm or injury.

            If they incline to think you dangerous To less than
            gods.                                 --Milton.

   3. In a condition of danger, as from illness; threatened with
      death. [Colloq.] --Forby. Bartlett.

   4. Hard to suit; difficult to please. [Obs.]

            My wages ben full strait, and eke full small; My
            lord to me is hard and dangerous.     --Chaucer.

   5. Reserved; not affable. [Obs.] ``Of his speech dangerous.''
      --Chaucer. -- {Dan"ger*ous*ly}, adv. --
      {Dan"ger*ous*ness}, n.

Source : WordNet®

dangerousness
     n : the quality of not being safe [ant: {safeness}]