The 'dec' part refers to the number 10 (ten). You may have heard of the word 'Percentile' (the 'cent' part referring to the number 100) which is used when looking at groups of 100 and establishing at what position a single person exists on that scale of 1 to 100.
It does not yet appear to be in the Oxford English Dictionery.
I have found many occurences of the word 'decile' in reports on children and their level of acheivement in education.
'Decile' appears to be a word created to describe a way of sub dividing groups, mailny children, based upon the students' ethnic origines. One report talks mainly of the Maori children (the original tribe of New Zealand) and their attendance at Low-Docile schools. This suggests that there are schools in existence that because of the low level of education and acheivement in that area, all students attending that school are described as belonging to the Low-Decile group.