DNLM stands for "屌你老母" (Diu2 Nei5 Lou5 Mou5-2) in Cantonese. It is a highly offensive phrase that literally translates to "f*** your mother" in English. This phrase is considered vulgar and should not be used in polite conversation.
La can mean Cantonese "啦". The pronunciation is "laa1," "laa4," or "laa6" depending on the context. ".la" is used here instead of the proper ".laa" simply because there's no ".laa" top-level domain available. That's the only reason. DNLM.
Wanna say DNLM to someone? Try this