To a very great degree or extent; enormously or immensely.
From Latin vastus meaning 'empty, desolate, immense' + English suffix -ly. The sense evolved from 'empty space' to 'great extent' by the 16th century.
The word 'vast' originally described empty, uninhabited places like deserts or wastelands, but gradually shifted to simply mean 'huge' - showing how words can lose their negative connotations over time!
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