This comes from the Latin decimare, meaning 'take the tenth man', and means 'kill or remove one in ten of'. It is now often used to mean 'damage severely', 'destroy or kill most of': Tuberculosis decimated the population; Cheap imports decimated the coal industry. Many people deplore this, both because of the meaning and because of the use of decimate with a single entity: you cannot destroy one in ten of the coal industry, although you may close one pit in ten. However, this usage is now very common, and seems likely to become accepted.
Yellow and the diamond shape represent Brazil's mineral wealth. The motto Ordem e Progresso means ‘Order and Progress’. Green stands for the vast forests. Effective date: 15 November 1889.