This study sought to explore how loanwords in Arabic are pluralized, whether loanword plurals are rule-based or idiosyncratic and the productivity of each type of loanword plural form: The Masculine Sound Plural (MSP), Broken Plural (BP), two or more Plurals, Zero Plural (ZP), Borrowed/Foreign Plural (BP) and Hybrid Plural (HP). It also aimed to shed light on the factors that contribute to the emergence of new, idiosyncratic plural forms. Analysis of a sample of 265 loanwords used in Standard and Colloquial Arabic collected from some Internet websites and glossaries showed that 37% of the loanwords in the sample have no plural forms (a Zero Plural) such as invariable singular nouns referring to food and drink (pizza, cappuccino), names of diseases and medications (bilharzia, Panadol), names of chemicals (potassium; ); sports (judo), sciences or areas of study (انثروبولوجيا anthropology), non-count nouns (ديزل diesel), Proper Nouns with a unique reference كونغرس) Congress ), noun referring to genus لافندر) lavender), and collective nouns (باغاج baggage). Results also showed that 35% of the loanwords take a BP only, whether it is a sole plural or a second or third plural form for the same loanword; 22% have double plurals as in nouns referring to some nationalities, ethnicities and religious groups, specialists and political loanwords; 16% have a BP (كادر /ka;dir/ > كوادر /kawa:dir/ cadres); 13% have a MSP together with another plural; 15% retain their Foreign Plural (يوتيوبرز YouTubers; موفيز movies; شيبس chips; داتا data); 4% have multiple plural forms (Malay (pl): الماليز (BP), ماليزيون (MSP), ماليزيات (FSP) and 1.5% have a HP (شوزات shoes+a:t/). Findings also showed that 46.5% of the loanwords follow the Arabic pluralization rules in the MSP, BP and FSP formation, even in the case of loanwords with double and multiple Plurals. Loanwords with a FP and HP represent a new morphological phenomenon in Arabic that was not reported by any prior study in the literature. The factors that contribute to the emergence of new idiosyncratic plural forms are discussed. Findings are also discussed in the light of some integration/adaptaion models of Arabic morphology. Recommendations for Arabic, English and translation pedagogy are given.