What's in a Name? Wat is in the Name

The city of Phnom Penh takes part of its name from the wealthy Khmer widow woman Penh, who founded a small monastery there.

It is situated at the confluence of the Tonle Sap and Bassac rivers with the Mekong, and it is said Penh found Buddha statues in a tree trunk on the banks of the river which in turn inspired her to found the monastery.

As the story goes, Grandma Penh (or Daun Penh) settled on a high land at the west bank of Chrap Chheam River (now Sap River next to Chaktomuk). At a vast flood season, there were many Koki trees floating to stick at Daun Penh high land and in the hole of a big Koki, there were four bronze and one stone Buddha statues.

In 1372, Daun Penh ordered people to pile up earth at northeast her house and used those Koki trunks to build up a pagoda on that hill for keeping the five Buddha statues, then named the temple Wat Phnom Daun Penh (now Wat Phnom)

The Cambodian word for hill is Phnom. Therefore the name of the town correctly translates as "Hill of Penh" and Wat Phnom means "temple built on a hill."

According to one Wikipedia:

The city takes its name from the Wat Phnom Daun Penh (known now as just the Wat Phnom or Hill Temple), built in 1373 to house five statues of Buddha on a man-made hill 27 meters high.

Phnom Penh was also previously known as Krong Chaktomuk (Chaturmukha) meaning "City of Four Faces". This name refers to the junction where the Mekong, Bassac, and Tonle Sap rivers cross to form an "X" where the capital is situated.

Krong Chaktomuk is an abbreviation of its ceremonial name given by King Ponhea Yat which was "Krong Chaktomuk Mongkol Sakal Kampuchea Thipadei Sereythor Inthabot Borei Roth Reach Seima Maha Nokor"

It first became the capital of Cambodia after Ponhea Yat, king of the Khmer Empire fled Angkor Thom when it was captured by Siam in 1431. There are stupas behind Wat Phnom that house the remains of Ponhea Yat and the royal family as well as the remaining Buddhist statues from the Angkorean era.