Superlative form of choicy, meaning most inclined to make careful choices or most selective in a picky manner.
From 'choicy' (choice + informal -y suffix) plus the superlative suffix '-est' (Old English). An informal construction for describing the most selective behavior.
In child psychology, 'the choiciest eaters' describes picky toddlers—it's a word parents use that admits their child has standards, even if those standards are frustratingly narrow and possibly just 'chicken nuggets.'
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