Pronounced to rhyme with "an archie".
Naartjie is a South African term (in South African English, it is a loanword from Afrikaans) used to refer to edible citrus fruit that are smaller than oranges, with bright orange loose skin with not too much pith. Inside are separatable segments that are sweet and tangy in taste.
You might call these tangerines, mandarins, clementines or satsumas (or even Citrus reticulata), but they are all naartjies. The word is sometimes also applied to mineolas and other recent hybrids, so long as they are small, orange, tangy and loose-skinned.
The root word "Naar" means sour or bitter in Afrikaans. It often carries a negative connotation, but not in this case. The "-tjie" suffix is a diminutive, thus a literal translation would be "little sour one" or "tangerino".
There is a South African chain of shops called Naartjie. They sell casual and funky children's clothing.
See also the legendary 1985 South African alt-protest-rock compilation on Shifty Records "A Naartjie in our Sosatie" (a sosatie is a kind of marinated kebab); featuring Bernoldus Niemand's "Hou my vas korporaal" (hold me tight, Corporal) and tracks by Sankomota and by the Kalahari Surfers.