knife-edge
From Longman Dictionary of Contemporary Englishknife-edgeˈknife-edge noun [singular] 1 UNCERTAINa situation in which the result is extremely uncertainon a knife-edge His future in the job is balanced on a knife-edge. a knife-edge vote2 WORRIEDa situation which makes someone very anxiouson a knife-edge Living with him, she is constantly on a knife-edge. the knife-edge of insecurity3 SHARPsomething that is narrow or sharp