นิล

From Sanskrit नील (nīla, “dark blue”); possibly via Old Khmer nīla, nila (“blue sapphire; shiny black, dark blue”). Cognate with Modern Khmer នឹល (nɨl); Lao ນິນ (nin), ນິລະ (ni la).

The last sense was coined by King Bhumibol Adulyadej around 1965 due to the fact that the term sounds like the first syllable of the fish's name in English, Nile tilapia, and that it reflects the colour of the fish's skin as well.

นิล • (nin)

1. jet:
   1. dense black coal used for jewelry.
   2. deep black.
2. any kind of black or very dark precious or semiprecious stone, including blue sapphire.
3. any colour that resembles the said stone.
4. (zoology) (classifier ตัว) (ปลา~) Nile tilapia: the fish Oreochromis niloticus of the family Cichlidae.