Question is pretty straight-forward, but why do Hong Kongers (don't know if others too) refer to Mainland China as daai6 luk6 ?
any historical reason for this or etymology?
It's due to the one-China policy (一中原則).
Both CPC (中國共產黨) and KMT (中國國民黨) think there is only one China. Both refer their own places as countries, PRC and ROC, respectively. Both treat the other side as an area or region, 台灣地區 and 大陸地區, respectively. At the beginning, KMT called the other side as 淪陷區. Because it is not a good term, KMT changed it to 大陸地區. 大陸 is a short form of 大陸地區. Many people in HK also moved from 大陸 due to the civil war (國共內戰). Therefore, these people tended to stand on KMT's side, and then used the KMT's term.
"大陸" as a term referring to china, or, "mainland china" had a long history.
there was an english newspaper "the china press" issued in shanghai, from 1911-1949; it's name in chinese is "大陸報".
the rationale of translating "china" to "大陸", my guess is: viewing from the settlement (租界), the land out of the boundary (aka china) is huge; so,
big - (大)
land - (陸 --> 陸地)
afterward, this term is still used in taiwan & hong kong, till nowadays, e.g.
陸委會 (大陸委員會) is mainland affairs council, an establishment in taiwan.
viewing from hongkong, with the same rationale, north of the "border", that "area" is huge; so the term 大陸 is a vivid, and valid description.
imo, "大陸" as a term for referring to china, is not due to any policy, or, events after 1949.