débil
ES [ˈdeβil] · Spanish for “weak, feeble” · sounds like Czech debil
Débil is an ordinary, polite Spanish adjective. It simply means weak. To a Czech ear though, it lands right on top of debil, a proper insult: moron, idiot. This opens the pathway to quite a few hilarious wordplays, for example the perfectly respectable periphrasis for “female” in Czech - něžné pohlaví (roughly “the gentle sex”) has its Spanish counterpart el sexo débil - “the weaker sex”. However, a native Czech hears something along the lines of “a person that is incapable of properly participating in an intercourse”.
The interesting part is that they’re actually relatives. Both come from Latin debilis, “weak.” Spanish kept the literal, physical sense; Czech narrowed it to “mentally weak” and then all the way to a schoolyard insult. One ancestor, two very different social registers.
As far as I can tell, a real faux ami: both apparently go back to Latin debilis (“weak”). Spanish kept the literal, physical sense; Czech seems to have narrowed it to “mentally weak” and then all the way to an insult. I'm no etymologist, but the shared root looks genuine — which is exactly why a neutral Spanish phrase can land so wrong on a Czech ear.