dum interim

From dum +‎ interim, both already meaning “while”.

dum interim (not comparable) (Late Latin)

1. during, while

   Synonyms: dum, interim, interea

   • [c. 400 CE, Donatus, Interpretationes Vergilianae, section 11,20:

     Interea, inquit, hoc est dum interim […]

     'Interea', he says, which means 'dum interim'^[1] […]]

In several descendants, the first syllable was reinterpreted as dē- and often deleted (having been taken as optional). It may be that there existed an early *minterim, inherited as such into multiple descendant branches, but the earliest attested Romance forms appear to be those that begin with do- or de-, with the syncopated forms surfacing only later.

• Joan Coromines, José A[ntonio] Pascual (1985) “mientras”, in Diccionario crítico etimológico castellano e hispánico [Critical Castilian and Hispanic Etymological Dictionary] (in Spanish), volume IV (Me–Re), Madrid: Gredos, →ISBN, page 70
• Pianigiani, Ottorino (1907) “mentre”, in Vocabolario etimologico della lingua italiana (in Italian), Rome: Albrighi & Segati
• Walther von Wartburg (1928–2002) “dum interim”, in Französisches Etymologisches Wörterbuch, volume 3: D–F, page 178