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®

dangerous
     adj 1: involving or causing danger or risk; liable to hurt or harm;
            "a dangerous criminal"; "a dangerous bridge";
            "unemployment reached dangerous proportions" [syn: {unsafe}]
            [ant: {safe}]
     2: causing fear or anxiety by threatening great harm; "a
        dangerous operation"; "a grave situation"; "a grave
        illness"; "grievous bodily harm"; "a serious wound"; "a
        serious turn of events"; "a severe case of pneumonia"; "a
        life-threatening disease" [syn: {grave}, {grievous}, {serious},
         {severe}, {life-threatening}]