Adopting an Optimality-theoretic approach, this paper examines gemination in English loanwords in Ammani Arabic (AA). Data come from a corpus of 1200 loanwords as produced by 12 AA monolingual native speakers. Results show that gemination, which is not attested in the source input, is induced to satisfy AA structural constraints and to render the output better well-formed. Of particular interest, results show that the introduction of English loanwords into AA highlights the activity of a constraint that requires prosodic words in AA, and probably many Arabic dialects, to be left-aligned with a foot. This constraint enhances our understanding of many aspects of Arabic phonology such as stress assignment and foot formation. The study has important implications for Arabic phonology, loanword phonology and second language acquisition.