S. #achar- v. “to avenge, do back, react, requite”
Conceptual Development: The initial element of this word was first derived from √AT “again” (PE17/166). Apparently the prefix ✶at- (also meaning “twice”) did not have the connotation Tolkien wanted, because he rejected this etymology, and went on to explore several different possible primitive forms meaning “back (again)”. Among these discussions, he coined the new root √AK, from which he derived the related Sindarin words acharn “(act of) revenge” and achared “vengeance”. The latter is clearly the gerund of achar-, so it seems this was also the new derivation of this verb.
References ✧ PE17/166-167
|√AT||“two, double, bi-, di-; back, re-; across, over, lying from side to side”||✧ PE17/166 (AT)|
|√AK||“hostile return”||✧ PE17/167|