From 宋·陈亮《戊申再上孝宗皇帝书》, where 陈亮 argued the 'barbarians' did not have a 方兴未艾之势 - a rising, undiminished momentum. Still used today precisely for momentum that hasn't peaked yet.
mistakes
艾 here means 'to stop/end' (archaic sense), not 'mugwort'. 未艾 = 'not yet ended' - the whole idiom is 'just rising, not yet ebbing'.
