This paper discusses idiomatic false friends (IFFs) in two genetically unrelated languages, English and Arabic. IFFs are defined as set phrases in two languages that have the same literal meaning but differ as regards their idiomatic meaning or their sociolinguistic and stylistic features. The study proposes a taxonomy for IFFs based on data from English and Arabic, though it may also apply to IFFs in other language pairs. In the case of English and Arabic, IFFs are either related (typically partial) or unrelated (typically total). Related IFFs have their origin in loan-translation, with idioms being borrowed from English into Arabic and then taking a different course of semantic development in each language. There are also cases in which the selection of a single sense of a polysemous idiom can be attributed to social and cultural factors. It is shown that, if idioms in general are among the most challenging units for translators, IFFs can be doubly difficult. The translator may assume that since the source and target language idioms have the same form, they can also have the same meaning or stylistic features.