Spanish flébil is a literary word that means the same as its Latin source flēbilis ‘lamentable, deplorable, mournful,’ which had come from the verb flēre ‘to weep.’ If no English relative comes to mind, it’s in part because the first l dropped out of flēbilis as the adjective evolved into the Old French feble that passed into English and has taken on the modern form feeble. Note that the shift in meaning is another reason English-speaking students of Latin wouldn’t make a connection to an English descendant of flēbilis. Once we accept feeble, we can pretty easily see that it has a doublet, foible. That’s the form that made it into early modern French but is now obsolete in that language. What had started out as an adjective came to be used as a noun to designate ‘the weaker part of a sword blade.’ Semantic expansion led to the modern sense ‘a minor weakness or peculiarity in someone’s character or behavior.’ Webster’s 1828 Dictionary gave this definition: “A particular moral weakness; a failing. When we speak of a man’s foible in the singular, which is also called his weak side, we refer to a predominant failing. We use also the plural, foibles, to denote moral failings or defects. It is wise in every man to know his own foibles.”
© 2018 Steven Schwartzman
Spanish-English Word Connections 
Jun 29, 2018 @ 21:10:59
Interesting how it was used in Old French and then discontinued, but it was assimilated in English as ‘feeble’ and ‘foible’. Great insight into the way ‘foible’ is used and its meaning, also being that of ‘the weaker part of a sword blade.’
Jun 29, 2018 @ 22:27:14
It’s interesting how a word can die out in one language but live on in another. I’ve come across other examples over the years. Here are a couple more involving French:
https://www.ahdictionary.com/word/search.html?q=polity
https://www.ahdictionary.com/word/search.html?q=linnet
Jun 29, 2018 @ 22:36:13
‘Polity’ is interesting, and see that what they use now is ‘politique’, similar to both English and Spanish ‘politics’ and ‘politica’.
Aug 11, 2018 @ 21:22:24
When I was growing up, ‘feeble’ and ‘foible’ referred to different things — at least, that’s how I remember it. The Iowa Institution for Feeble-Minded Children, established in 1876, had undergone a name change by the time I came along, but ‘feeble-minded’ still was a phrase in use when I was a kid; it referred to those who were “weak in the head” — another phrase I remember hearing.
On the other hand, ‘foible’ wasn’t so much a moral failing or defect as a quirk: harmless, and occasionally charming. I suspect distinctions were made, of course: e.g., I have foibles, she has character flaws.
I rarely hear ‘feeble’ any more. Occasionally I’ll hear a reference to a feeble argument, or a feeble attempt to do this or that, but it’s rare. When I went over to the ngram viewer to see what’s been happening with ‘feeble’, my suspicions were confirmed. The use of ‘feeble’ has been plummeting since about 1860. Perhaps some of the documented horrors in places like the Iowa Institution contributed to the slow demise of the term.
Aug 11, 2018 @ 21:52:50
I’m likewise old enough to remember the phrase feeble-minded. Maybe because faible remains a common word in French I hadn’t paid attention to the declining fortune of feeble in English.
Also like you, I think of foible as more of a quirk than a moral failing. The Oxford Living Dictionaries definition mirrors that: ‘A minor weakness or eccentricity in someone’s character.’