late 14c., ascendent, in astrology, "rising over the horizon," from Latin ascendentem (nominative ascendans), present participle of ascendere "to mount, ascend, go up" (see ascend). Sense "moving upward, rising" is recorded from 1590s.
As a noun in astrology, "point of the ecliptic or sign of the zodiac which is on the eastern horizon at the moment of birth." The planet that rules the ascendant is believed to have predominant influence on the horoscope. Hence in the ascendant "ruling, dominant" (not, as is often thought, "rising"), 1670s, and the adjective meaning "superior, dominant," 1806.