شرموطة ‹šarmūṭa› is one of the most taboo curse words in the Arabic language. It means ‘whore,’ but it is much more potent as an insult. Speaking from personal experience, I very rarely hear it.
Linguistically, it has an unusual structure for an Arabic word. Its root is composed of four consonants š-r-m-ṭ, while most roots are composed of three. Its word pattern |C₁aC₂C₃ūC₄a| is also relatively rare. Perhaps because of how taboo and structurally unusual it is, ‹šarmūṭa› has been the subject of some highly imaginative and very wrong etymology theories.
The entry in the English Wiktionary claims, “Most Arab linguists agree that it is of non-Semitic origin.” The Arabic Wiktionary entry, after giving the correct explanation, provides the most common folk etymology: that it comes from the French charmante, meaning “charming, delightful.” The story is that during the colonial period, French soldiers would call Arab girls who flirted with them charmante. The local population, who could not pronounce it correctly, mistook the word for something dirty given the improper behavior of these women and therefore interpreted it as ‘whore.’
The Arabic Wiki entry goes on to claim that charmante is also the origin of Sharm, as in Sharm el Sheikh, the Egyptian resort town. That factoid is indisputably false because “sharm” is a perfectly ordinary Arabic word meaning “bay.” Even leaving that aside, there is no plausible explanation for how charmante — with the nasal /ɑ̃/ vowel in the second syllable — became ‹šarmūṭa› with a long /uː/ vowel. Compare the Arabic colloquial word for “elevator” ‹aṣansēr›, which is a direct borrowing from French ascenseur. The en of the second syllable — identical in pronunciation to the an in charmante — does not transform into a long /uː/ vowel, even among non-French-literate Arabic speakers. The charmante explanation is nonsense, most likely the result of someone noticing that it sounds somewhat similar to ‹šarmūṭa› and deciding that both words must have a common (French) origin.
The actual etymology is so fantastically convoluted that the charmante theory is almost an insult by comparison. The original root of ‹šarmūṭa› is actually triliteral š-r-ṭ, which means ‘slice, tear off.’ Recall from my post on Semitic languages that Arabic, being a Semitic language, has a root-pattern system of word formation. Basic verbs are formed with the pattern C₁aC₂aC₃-; applying the root š-r-ṭ to this pattern results in ‹šaraṭ-› ‘to slice, tear off.’ The passive participle of that verb form has the pattern maC₁C₂ūC₃-, and the resulting word is ‹mašrūṭ-›, meaning ‘sliced, torn off.’ At some point, through a type of sound change called metathesis¹, where the sounds of a word are rearranged, ‹mašrūṭ-› became ‹šarmūṭ-›.
In Arabic, passive participles are adjectives, and adjectives can be nouns. So ‹mašrūṭ-›/‹šarmūṭ-› meant not only ‘sliced, torn off’ but also ‘a thing that is sliced or torn off.’ The feminine form of this word ‹šarmūṭa› came to mean ‘rag’ (as in a torn piece of cloth), specifically one used to wipe up dirt². By metaphor, this word then morphed into an insult against women. Calling a woman a ‹šarmūṭa› was saying that she was as “dirty” (i.e. in terms of morals and reputation) as a dish rag. Thus was born one of the worst insults in the Arabic language.
‹šarmūṭa› is not French nor Ancient Egyptian as someone hilariously tried to claim. No, it is a purely Arabic word, although one with a very exceptional journey.
¹ This rearranging of sounds happens in all languages. For example, in English, “aks” is a nonstandard variant of the word “ask”, both of which forms go back to Anglo-Saxon; and in Spanish, milagro ”miracle” comes from Latin miraculum.
² A Saudi twitter user reports that in certain parts of Saudi Arabia, the word for rag is ‹šamṭūr›, which is a metathesis of the already metathetic ‹šarmūṭ›! This second metathesis may well have resulted from a need to separate the original meaning of the word from the now vulgar usage.