As an adverb, very is used to make an adjective or adverb stronger, like in 'very big' or 'very quickly'. As an adjective, especially in older or formal English, it means exact or real, as in 'the very same day'.
From Old French 'verai', meaning 'true or real', from Latin 'verus' meaning 'true'. Its meaning softened from 'truly' to a general intensifier in everyday English.
‘Very’ started out meaning ‘truly’, so ‘very good’ was basically ‘truly good’. Over time it weakened into a filler word, which is why teachers often tell you to replace it with something stronger. The history of ‘very’ is a lesson in how words can fade from powerful to pale.
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