People who are ungrateful; those who fail to appreciate kindness or favors shown to them.
From Latin 'ingratus,' combining 'in-' (not) with 'gratus' (grateful, pleasing). The root 'gratus' also gives us 'grace,' 'grateful,' and 'gratitude,' all sharing the idea of appreciation.
Ingratitude is considered one of the worst character flaws in literature and philosophy—Shakespeare, Dante, and countless others wrote about how ungrateful people are worse than enemies because they betray trust, which is why the word carries such moral weight.
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