Izquierdo
A Spanish surname from a nickname meaning “left-handed” or “on the left side.”
The #5,791 most common surname in the United States (2010 Census) — about 1 in every 51,682 Americans shares it.
Etymology
Izquierdo comes from Spanish izquierdo, “left” or “left-handed.” The word was used as a descriptive nickname for someone who was left-handed, had a left-side feature, or was associated with the left side in a physical or social sense. In older usage, it could also carry the sense of “awkward” or “crooked,” reflecting long-standing cultural associations with the left side.
History
The sources you provided treat Izquierdo as a nickname-style surname and gloss it with senses such as “left,” “crooked,” and “left-handed.” It is a well-established Spanish surname and remains especially common in Spain and across Spanish-speaking Latin America. As with many descriptive surnames, it likely arose independently in multiple places from the same everyday adjective.
Origin
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Fun fact
Spanish izquierdo is etymologically connected to the idea of the left side, but in many historical contexts “left” and “sly/awkward” could overlap in meaning and connotation.
From the historical record
“Izaga. El lugar donde hai muchos juncos. Izquierdo. Adjetivo que se aplica a los miembros dobles del animal que caen al lado del corazon. Torcido, Zurdo, Siniestro. J”
— Diccionario de los apellidos