A shortened version of the expression "butt cheeks". The expression "butt cheek" should only be used in brief exchanges accompanied by snickers (the laughter not the candy bar). However the expression "butt cheeks" can be used indiscriminately by such highly-educated types as doctors, lawyers and Uncyclopaedic editors.
It is claimed by people in the know (see also Grand Conspiracy), that the expression originated in the ancient Greek saying butteus et cheekiostimus, which loosely translated means "Oh my god Sarah, her butt is so fat!". However Russian linguists claim that they found it first.
The expression "butt cheek" appears in a 7th century tribute to Queen Boedica also known as "Her Majesty Rotunda". We next find it used in a 10th century medical work, detailing the various ways in which leeches might be attached to the posterior for bleeding. However the expression did not come into wide usage until it was popularized in a 12th-century ballad by Henri de Molluskcula called "Baby got Back"