It means "moron/moronic" in a few other languages too. Check out: https://en.wiktionary.org/wiki/debil
The word comes from the Latin word debilis (weak, feeble, frail), but it's interesting how some languages (Spanish, Portuguese) developed words from the root to refer to physical weakness and some others (Albanian, Czech, etc) to refer to mental weakness.
It's nice to see that Czech and Croatian have the same meanings here, since we have a lot of similar words with the opposite meanings. For example, Croatian "divan" (wonderful) is in Czech "úžasný", while in Croatian "užasni" are people who are - terrible. :D Thank you for your explanation. :)
I can see where “divan” can come from in Slavic languages (in Russian — “divo” is one of the words for “miracle”), but in Russian “divan” means “sofa”, the word is of Turkish origin (same in English). “Horrible” is the same in Russian (ужасный/uzhasny). :)
And yes, we use the word “debil” as a slur (originally, from the medical term “mentally debilitated”).
I looked it up and actually, "divan" as an adjective (wonderful) comes from the verb "diviti se" (to admire) and it came to Slavic languages directly from Latin "deus" (God). It also has a different accent "dȋvan". Turkish/persian for sofa, however, is "dìvān". We have it in Croatian colloquial language but it is limited to some villages (probably all of them in Slavonia region). They addopted Turkish dìvān, but not as a "sofa", but as a "conversation". There is also a verb "dìvāniti" (to talk, communicate). But, today you can rarely hear "dìvān", it is used only by older people.