Tim Bowen sheds some light on the origins and definition of the term to earmark.
“The European Union has announced that it intends to earmark further funds for transportation”. In this sense the verb earmark means ‘allocate’, ‘designate’ or ‘set aside’.
Like many expressions, the word appears to originate in the world of animal husbandry. Before the days of branding cattle with a hot iron to identify who they belonged to, farmers would cut the ears of their animals in a particular way to set them aside from other animals in common grazing areas. An earmarked animal would therefore be clearly identifiable to its owner. The idea of ‘setting aside’ or ‘separating’ has remained with the word in its modern-day usage.