It is pronounced ob-dor-MISH-uhn. It means
numbness in a limb, usually caused by pressure on a nerve. Also
known as falling asleep. From Latin obdormire (to fall asleep),
from dormire (to sleep). Earliest documented use: 1634.
Incidentally, there is a good word that comes after
obdormition: paresthesia, known as pins and needles.