expression, godan verb (-ru)
harden one's heart; steel oneself; be ruthless
Idiomatic expression meaning to suppress emotions and act harshly or mercilessly, often for a necessary purpose. The image is of becoming a demon (鬼).
上司は鬼になって、ダメな部下を叱った。
The boss hardened his heart and scolded the incompetent subordinate.
親は時には鬼にならなければならない。
Parents sometimes have to be ruthless.
expression, godan verb (-ru)
become 'it' (in tag, hide-and-seek, etc.)
Used in children's games to mean becoming the chaser or seeker. The demon (鬼) is the 'it' role.
鬼ごっこで、最初に鬼になるのは誰？
Who's going to be 'it' first in tag?