'pirarucu' originates from Portuguese, specifically the word 'pirarucu', where the elements come from Tupi: 'pirá' meant 'fish' and 'ruku' (or 'uruku') meant 'large' or 'big'.

'pirarucu' was adopted into Portuguese from the Tupi language of Brazil and later entered English usage (via Portuguese) in the 19th century as the name for the large Amazonian fish now identified as Arapaima gigas.

Initially it meant 'large fish' in the source language context, but over time it became the specific common name for the species now called Arapaima gigas.