Almagest (n.) Look up Almagest at Dictionary.com
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" (see mickle).

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,