Short answer:
Nothing wrong with it at all!
I recently met a good friend (Turkish) and said “Hi, how’s you?”. He went on a rant at me saying how it’s not correct English, I should know better because I am an English teacher, do you teach that to your students? … blah, blah, blah!
Is it correct grammatically? NO! The expanded saying would be “How is you?” which is obviously incorrect, but it is so widely used in the UK that it has become acceptable. Cambridge dictionary has it entered, Urban dictionary gives some background.
UK spoken English
I must admit that it is a spoken phrase for UK only, American’s will understand, but not use it.
English is not a grammar based language, native speakers make mistakes all the time and nobody cares! Many other languages are grammar based so when learning English, they focus on grammar. Whilst learning the grammar is important, it should not be the main focus.
Relax, English is very flexible
When I begin to think of all the things I say or have been told by mother tongue/native speakers, most pure English teachers’ blood would boil! Do you understand what is meant? Does it really matter so much? Fluency, communication and understanding are the keys to any language, English is just more liberal in that interpretation than most!
Furthermore, in this instance, it is actually an entry in the dictionary as spoken English, so you could argue that it is correct English. There are thousands of such entries, far too many to list, also depending on the country of use. This is maybe frustrating for some people, but I love the flexibility it provides to be creative.
Do you have any examples?
Are there any you love/hate?
Let me know in the comments