English, French, Spanish, Portuguese, Dutch, etc.: from the Latin personal name Lucas (Greek Loukas) ‘man from Lucania’. Lucania is a region of southern Italy thought to have been named in ancient times with a word meaning ‘bright’ or ‘shining’. Compare Lucio. The Christian name owed its enormous popularity throughout Europe in the Middle Ages to St. Luke the Evangelist, hence the development of this surname and many vernacular derivatives in most of the languages of Europe. Compare Luke. This is also found as an Americanized form of Greek Loukas.
Scottish: reduced Anglicized form of Gaelic Mac Lùcais (see McLucas). source