Which word would describe the act of flattering someone excessively?

The word "adulation" precisely captures the essence of excessively flattering someone. It refers to insincere praise or an excessive display of affection and admiration, often aimed at gaining favor or approval from the person being flattered. This term conveys the idea that the flattery is not just generous, but rather exaggerated to the point of being sycophantic or overly fawning.

In contrast, "assiduous" describes someone who is diligent and hardworking, which does not relate to flattery. "Acerbic" refers to a sharp or harsh manner of speaking, particularly when it involves criticism, while "ascetic" pertains to a lifestyle characterized by self-denial and austerity, often for religious or spiritual reasons. None of these terms connote the idea of excessive praise or flattery as "adulation" does, making it the most fitting choice for describing the act in question.