I had to learn the real meaning in graduate school. My husband is having some problems along those lines.
However, in the spirit of WoD, here �s my "daffynition":
sterno: a gel-like fuel used to keep food hot on a serving line
cleido: from the Greek for closed
mast: a pole on a ship used to hold the sails
oid: like
eus: an abbreviation for endoscopic ultrasound
So, obviously, sternocleidomastoideus is checking the inside of a
closed pole or tube using ultrasound in order to see if there is any
sterno contained within. Easy!