Annoyed, disappointed, or unhappy because expectations weren't met.
From 'dis-' (a prefix meaning 'not' or 'opposite of') combined with 'gruntle' (from Old English, meaning 'to grunt' or 'to make a low sound of satisfaction'). So literally means 'not satisfied.'
The word 'gruntle' (meaning to satisfy) almost disappeared from English, but the negative 'disgruntled' became so common that people now sometimes jokingly use 'gruntled' to mean the opposite, showing how language evolves through back-formation.
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