3. sudden, distressing, violent. 4. keen, astute, discerning, perceptive, intelligent, perspicacious; sharp-witted, clever, smart, bright, ingenious, brilliant; knowing, wise, sage, sagacious, sapient. Acute, penetrating, shrewd imply a keenness of understanding, perception, or insight. Acute suggests particularly a clearness of perception and a realization of related meanings: an acute intellect. Penetrating adds the idea of depth of perception and a realization of implications: a wise and penetrating judgment. Shrewd adds the idea of knowing how to apply practically (or to one's own advantage) what one perceives and understands: wary and shrewd. 5. keen.