Is it ensure, insure, or assure?
1847 (calisthenic (adj.) is from 1839), formed on model of French callisthenie, from Latinized comb. form of Greek kallos "beauty" + sthenos "strength" + -ics. Originally, gymnastic exercises suitable for girls and meant to develop the figure and promote graceful movement. The proper Greek, if there was such a word in Greek, would have been *kallistheneia.
calisthenics cal·is·then·ics (kāl'ĭs-thěn'ĭks)
Gymnastic exercises, such as sit-ups, designed to develop muscular tone and promote physical fitness.