What does Garrett mean in cockney slang?

In Cockney rhyming slang, "Garrett" rhymes with "cigarette". Therefore, "Garrett" is slang for "cigarette".

This is an example of how Cockney rhyming slang works: a two-word phrase is used to replace a single word, and the two words rhyme with the original word.

Keep in mind that Cockney rhyming slang is very playful and often used for humorous effect.