1580s as a command to get attention, in which use it belongs in the group with hello, hallo. From 1520s as a command to "stop, cease," from French holà (15c.), which "Century Dictionary" analyzes as ho! + la "there." As an urban slang form of holler (v.) "greet, shout out to," it was in use by 2003.

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Definitions of holla from WordNet

holla (n.)
a very loud utterance (like the sound of an animal);
Synonyms: bellow / bellowing / holler / hollering / hollo / holloa / roar / roaring / yowl