to make something deeper or to increase in depth.
From 'depth' (Old English 'deop') plus the suffix '-en' meaning to make or become. The verb form was created by adding a productive English suffix to the noun.
This is a great example of how English speakers can turn almost any noun into a verb by adding '-en'—we can 'darken,' 'deepen,' 'strengthen'—it's like having a built-in word factory for creating action words from descriptions!
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