late 14c., title of a treatise on astronomy by Claudius Ptolemy of Alexandria, from Old French almageste (13c.), from Arabic al majisti, from al "the" + Greek megiste "the greatest (composition)," from fem. of megistos, superlative of megas "great" (from PIE root *meg- "great").
Originally titled in Greek Mathematike syntaxis ("Mathematical Composition"), commonly called Megale syntaxis "Great Composition" (Greek megale is the fem. of megas); Arab translators in their admiration altered this. Extended in Middle English to other works on astrology or astronomy,