Either Both/Or

Someone wrote me to challenge the use of “either” to mean “both” as in phrases like “on either side.” We’re more used to thinking “either” as meaning “one or the other” as in “you could take either the high road or the low road.”

According to the Oxford English Dictionary, the oldest meaning of "either" is "each of the two," and the first recorded instance of the use of the word in writing is from 10th-century Anglo Saxon:

Hwa is þætte ariman mæge hwæt þær moncynnes forwearð on ægðere hand [“on either hand”].

Here's an example from Middle English, 1325:

Þe holi strem of flum iordane On aeiþer side [“on either side’] stude still as stane. {“The holy stream of the River Jordan on either side stood still as stone.”]

Here’s an 1819 example from Sir Walter Scott:

There was a huge fire-place at either end of the hall.

“Either” meaning “One or other of the two” appears almost three centuries after this meaning.

The original meaning of the word is preserved in traditional phrases like "on either hand" and "on either side."