Noun
A person who maintains railway lines.
Someone whose job is to sand and grind small imperfections from metal and ceramic castings.
(by extension) Someone who fiddles or tinkers with things.
(slang, UK, Lancashire) A friend or mate
How's tha' doing fettler?
(mining, historical) An ostler; one who looks after the horses in a mine.
Source: en.wiktionary.org