Sadly, I so enjoy sweeping along in Patrick O’Brian books that I often fail to note and look up the words I do not know (another good use for BookDarts!) Today, however, I decided I had to look up ataraxy, if only out of shame for having forgotten to mark, look up or even correctly recall something like ‘vaticination’ a chapter or so earlier.

Merriam-Webster hides the word behind its pay-wall, so I had recourse to the Wiktionary. The first example in the definition is, hilariously, the sentence that lies over my knee in HMS Surprise: “There was no longer any need for fortitude: he felt nothing at present and there was no point in artificial ataraxy.”

Without further ado then:
ataraxy: sullen indifference, or imperturbable calm.

Given the laudanum-related context, the latter seems more likely.