There is a site learn you a haskell with the title "Learn You a Haskell for Great Good!". Does "Great Good" mean "very very good"? Does the whole phrase mean "learning Haskell is good for you" or "learning Haskell is good for the whole world" or something else?
Haskell is a very mathematical programming language. It is typically learned by mathematically inclined people who have big brains. That makes the subject matter frightening to many people. It triggers their "math anxiety".
The title is deliberately silly so as not to make people feel anxious. Haskell books are typically pedantic and formal. The informal, grammatically incorrect, silly style of the title is kind of promise to the reader that the book won't be overly pedantic and formal.
I think also that the author, who is Slovenian, may be making fun of his own English skills. (The English inside the book however is very good.)
In better English, the title might be something like, "Learn Haskell for great benefit".
I'm inclined to think it's a parody of "For great justice!" from the badly-translated game, Zero Wing.
According to this post the phrasing originated in the title of the Haskell tutorial "Write Yourself a Scheme in 48 hours", written in 2006, and was simply a (not that ungrammatical, but certainly awkward) "portmanteau of two different common memes at the time": the "Teach yourself X in 21 days books", and Peter Norvig's semi-parody "Teach yourself programming in 10 years".