Definition of plunge into
1 : to push (something) into (something) quickly and forcefully He plunged the knife into the cake. I plunged the sponge into the bucket of water.
2 : to start doing (something) with enthusiasm and energy She plunged (right) into the assignment.
3 : to suddenly begin to be in (a particular and usually bad condition or situation) He plunged into a severe depression.
4 : to cause (someone or something) to suddenly be in (a particular condition or situation) A series of bad management decisions had plunged the company into debt. The author plunges his readers into a world of fantasy.
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