débil
ES [ˈdeβil] · Spanish for “weak, feeble” · sounds like Czech debil
Débil is an ordinary and polite Spanish adjective, meaning weak. To a Czech ear though, it lands right on top of debil, which is 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.
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.