While writing an answer to this question, I looked up the word ennui in the full version of the Oxford English Dictionary. (I'd give you a link, but I access the OED through my local library's proxy service, so it wouldn't work for you; if you have access, go to oed.com and search for "ennui" as a noun). Three senses are given. The first and third senses are normal dictionary definitions with the customary example sentences, but the second, in its entirety, is:
What does this mean? I'm not asking about the word per se—yes, I know what personification is—I'm wondering why the word appears alone here, without elaboration or examples or a clue as to how it pertains to the rest of the entry.
A possible clue: Sense c ("A cause of ennui") is accompanied by four example sentences, the first two of which contain personifications of ennui:
1790 C. M. Graham Lett. Educ. 290 It would entirely subdue the dæmon Ennui.
1812 H. Smith & J. Smith Rejected Addr. 11 The fiend Ennui awhile consents to pine.
Have these two examples simply been attributed to the wrong sense, or is something else going on here? Does anyone have access to the print version to check?