The superlative form of 'beety,' meaning having the most beet-like qualities or containing the most beets.
From 'beet' (a root vegetable) plus '-y' (meaning 'characterized by') plus '-est' (superlative ending), following the standard English pattern for intensifying adjectives.
The fact that 'beetiest' exists means someone, somewhere, actually needed to compare vegetables by how much they taste or look like beets—probably a chef or gardener debating whose beet crop was most 'beet-like.'
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