The origin of the name of neighbouring Long Reef is obvious, but the reasons for Dee Why's name remain unclear. The earliest reference to it is a pencil note in surveyor James Meehan's field book, "Wednesday, 27th Sept, 1815 Dy Beach - Marked a Honey Suckle Tree near the Beach".[4] What it meant to him was not recorded[5], although various claims have been put forward, including:

The letters DY were simply a marker that Meehan used to mark many other places on his map.[6]

The name came from the local Aboriginal language that Meehan used to name many of the locations that he surveyed.[7]

From 1840 the name was recorded as one word, but was split in two during the 1950s. The term 'Dee Why' was also used to name 'Dee Why Heights' or Highlands, known as Narraweena since 1951, and 'Dee Why West', the name of which was changed to Cromer in 1969. [6]

Little is known of the Aboriginal people who lived in the Dee Why area before European occupation, although there is evidence of a midden at the southern end of Dee Why Beach, and the indigenous people were known to fish on the then wider and deeper lagoon, where black swans were once seen in large flocks.[7]

Early development
The first land in the area to be listed by the New South Wales government Gazette was 700 acres (280 ha) granted to William Cossar in the early 19th century, but by the mid 1800s most of the land in what is now Dee Why had been acquired by James Jenkins and other members of the Jenkins family.[7] Elizabeth Jenkins, eldest daughter of James, gave all her land to the Salvation Army upon her death in 1900, in recognition of their support in her old age. The Salvation Army received in total 1,740 acres (700 ha) of land, 200 of which were in Dee Why. An industrial farm, as well as hostels for boys, girls and women were established on this land. Access to the beach was limited by the Salvation Army's land, a wire netting barrier ran along its length, but this changed when in 1913 the Salvation Army decided to progressively sell off most of its holdings on the Northern Beaches, starting with The Oaks Estate Auction. The main street of Dee Why, Howard Avenue, commemorates Commissioner Thomas Howard, the first commissioner of the Salvation Army in Australia and New Zealand. By 1920, most of Dee Why had been subdivided.[8] Source: wikipedia