The "mashie" was the historical golf club (wooden-shafted, mostly pre-20th Century) that most closely resembled today's 5-iron. A modern 5-iron and the mashie are very different clubs; it would be incorrect to call a modern 5-iron a mashie. But in loft and use, the mashie was most equivalent to a modern 5-iron.
Source: British Golf Museum
Common Misspellings: Mashy, mashee