From Longman Dictionary of Contemporary Englishswitchbladeswitch‧blade /ˈswɪtʃbleɪd/ noun [countable] a knife with a blade inside the handle which springs out when you press a button
Examples from the Corpus
switchblade• He once pulled a switchblade on his director because they couldn't agree on the way to do a scene.• Lucy looked down and realised that she was still carrying the switchblade.• I pulled the switchblade out of my pocket and pressed the button.• The switchblade fell from his hand as he sagged to the ground, whimpering softly, his hands clutched between his legs. 