It seems like it's a grammatically wrong phrase but it is actually Koreanized English term which means "When I was your/at your age" or "during my time". Since "나 때(Na ddae)" sounds like "Latte" and "말이야(Mal ee yah)" sounds like "is (a) horse", Koreans translated these into English in their own way and using the phrase in a daily basis. It is a common phrase used among boomer generations.

The phrase can be used in a satiric way in a certain situation like below.

A term used to reference cannabis/marijuana/weed.