Origin: L sestertius (nummus), for semis tertius, two and a half, because equal in value to two and a half asses
See sesterce in American Heritage Dictionary 4
Origin: Latin sēstertius, a coin worth two and a half asses
Origin: : sēmis, half; see sēmi- in Indo-European roots
Origin: + tertius, third; see trei- in Indo-European roots.