The term mastro (i.e. mastro-Michalis) is an honorific used in Greece to designate a craftsman or a mechanic as having achieved a high level of skill in his profession.
It probably originates from the mediaeval European guild designation of Master Craftsman which in term originates from the Latin magister. Today, in modern Greece, it is usually applied to an older person who after a long apprenticeship and professional practice has gained the respect of his fellow professionals. On rare occasions it is also applied to an exceptional younger professional.