Why is the ninth month called September?
early 15c., from Middle French assister "to stand by, help, put, place, assist" (14c.), from Latin assistere "stand by, take a stand near, attend," from ad- "to" (see ad-) + sistere "take a stand, cause to stand," from PIE *siste-, reduplicated form of root *sta- "to stand" (see stet). Related: Assisted; assisting. Medical assisted suicide attested from 1884.
1570s, "an act of assistance," from assist (v.). In the sporting sense attested 1877 in baseball, 1925 in ice hockey.