I’ve used the phrase “in parenthesis”—never mind having read it thousands of times—without becoming aware of the fact that the word already contains an in. In what follows my source is Online Etymology Dictionary. It tells me that the three roots of the word are “beside” (para-), “in” (en), and the Proto-Indo European “put in place” (thithenai). Thus it originally signaled an action—the putting in of something, a letter or a syllable, beside, betwixt, between something else textual. Thus that which we put between parentheses, is already something “put in.”
Now this on the surface absolutely pointless exercise in etymological investigation—after all the word now means a marker that begins or ends an “aside,” a kind of secondary or whispered addition to text, a kind of “you might be too stupid to know, but here is what it means, chum; but since I don’t want to insult you, I’ll make it easier to skip—if you are bright enough to know what those marks mean”—this pointless exercise arose from a medical day in this household or, more precisely put, a medical twenty-four hours. One of us had to spend that time in hospital; the other was the Accompanying Fellow Sufferer (without actual pain). This experience, thankfully over now, reminded me of a very troubling, very powerful book I’d read in my youth. It was David Jones’ In Parenthesis. The book appeared in 1934 and was a recollection of the horrors of World War I. Jones’ title makes use of yet another meaning inhering in parenthesis: that of separation, that of holding apart. Some of our experiences are of such a nature that we do not wish to dwell on them. We’d just as soon put them between parentheses. Jones separated his experiences using his title—but told us all about them nonetheless, thus illustrating the altogether ambiguous nature of this separation in some cases.