The official pronunciation would be something like 'eh-tche-ray'.
'ecere' is the feminine dative form of 'ece' (eternal) in Old English.
http://en.wiktionary.org/wiki/ece#Etymology_2
Also see second page (120) of http://ir.tokyo-kasei.ac.jp/metadb/up/k ... k_0703.pdf
A derived more modern but obsolete English word would be http://en.wiktionary.org/wiki/eche#English
However since we found out about this meaning years after adopting it we still often pronounce it 'ee-seer' ourselves
Cheers,
-Jerome