Hesychius of Alexandria in Wikipedia

Hesychius of Alexandria (῾Ησύχιος ὁ Ἀλεξανδρεύς), a grammarian who flourished probably in the 5th century CE, compiled the richest lexicon of unusual and obscure Greek words that has survived (in a single 15th century manuscript). The work, titled "Alphabetical Collection of All Words" (Συναγωγή Πασών Λέξεων κατά Στοιχείον), includes approximately 51,000 entries, a copious list of peculiar words, forms and phrases, with an explanation of their meaning, and often with a reference to the author who used them or to the district of Greece where they were current. Hence, the book is of great value to the student of the Greek dialects, while in the restoration of the text of the classical authors generally, and particularly of such writers as Aeschylus and Theocritus, who used many unusual words, its value can hardly be exaggerated. Hesychius is important, not only for Greek philology but also for studying lost languages and obscure dialects (such as Thracian and the ancient Macedonian language) and in reconstructing Proto-Indo-European. Hesychius' explanations of many epithets and phrases also reveal many important facts about the religion and social life of the ancients. In a prefatory letter Hesychius mentions that his lexicon is based on that of Diogenianus (itself extracted from an earlier work by Pamphilus), but that he has also used similar works by the grammarian Aristarchus of Samothrace, Apion, Heliodorus, Amerias and others. Hesychius was probably not a convert to Christianity. Explanations of words from Gregory Nazianzus and other Christian writers (glossae sacrae) are later interpolations. The lexicon survives in one deeply corrupt 15th century manuscript, which is preserved in the library of San Marco at Venice, (Marc. Gr. 622, 15th century). The best edition is by Moriz Wilhelm Constantin Schmidt (1858-1868), but no complete comparative edition of the ms has been published since it was first printed by Marcus Musurus (at the press of Aldus Manutius) in Venice, 1514 (reprinted in 1520 and 1521 with modest revisions). Under the auspices of the Royal Danish Academy of Fine Arts in Copenhagen a modern edition has been in intermittent publication since 1953: to date, the new edition covers alpha through tau.