To shout or yell loudly; also a deep valley or hollow between hills, especially in mountain regions.
From 'hollow' with regional pronunciation shift; 'holler' as a valley comes from Dutch or English 'hol' meaning 'hollow'; as a verb it developed from the noun, meaning to shout into a hollow.
In Appalachian English, 'holler' (hollow) refers to the narrow valleys where families settled, and the shouting meaning comes from how sound travels down those valleys—people actually did holler to communicate across distances.
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