It stands for the Dalai
Lama, the spiritual head of Tibetian Buddhism in
Cantonese. The same word is also used to represent a
foul word in Cantonese meaning "I Fucked Your Mum".
Teacher: Swearing is not allowed in the classroom, students!
Student: Dllm 8745
Teacher: What did you say?
Nerd: Teacher, didn't you know that the Dalai Lama is the spiritual head in the Tibet Autonomous Region?