To an extent that is more than necessary, normal, or reasonable. In a manner that goes beyond appropriate or acceptable limits.
From Latin excessus meaning 'departure' or 'going beyond,' formed from ex- 'out of' and cedere 'to go.' The concept evolved from physical departure to metaphorical transgression of boundaries or limits by the 14th century.
Interestingly, what constitutes 'excessive' is highly subjective and culturally determined - Victorian society considered showing ankles excessive, while modern standards would find that quaint. The word often reveals more about the speaker's values and boundaries than about the behavior being described.
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