In notes from around 1959 Tolkien had a verb akkar- or ahtar-, cognate to S. achar- “do back, react; requite; avenge” as combination of √AT “back” (related to √AT-TA “two”) and √KAR “do” (PE17/166). This etymology was rejected when Tolkien decided that, as a prefix, at- meant “re- of the same or other agent in repeating (more or less similarly) a previous action” and in Q. “only in literal sense of second time, double”. It was thus not suitable for “avenge”.

Neo-Quenya: For purposes of Neo-Quenya, I think ᴺQ. ahtar- can be salvaged in the more limited sense “to do back, react”.