Like the vast majority of English speaking people my knowledge of Arabic, for this is certainly an Arabic expression, is purely aural but may I suggest that it's an Anglo-Saxon corruption of two words. Arabic “kiffak”, or more formally “kif hallak” meaning thanks, and “Allah”. So “Allah kiffak” means “thanks be to God”; an expression heard more commonly in English 70 years ago. Nowadays “Insh' Allah” is more often used.