The specific epithet inermis is Latin for "unarmed" and refers to the upper pitchers of this species, which are unique in that they completely lack a peristome.

[2][note d] This final specimen, Bünnemeijer 9695, was later designated as the lectotype of N. inermis by Matthew Jebb and Martin Cheek.

[note e] He wrote: "This new species is easily distinguishable from all others by the peculiar pitchers without peristome and with very narrow lid.

"[2] Several of the specimens collected by Bünnemeijer were labeled with the local vernacular names galoe-galoe antoe and kandjong baroek.

[2] In 1986, Mitsuru Hotta and Rusjdi Tamin included plant material belonging to N. dubia and N. inermis in their description of N. bongso.

[3][7] In a 1993 study of Nepenthes prey and pitcher infauna,[8] the same authors, together with M. Kato and T. Itino, identified N. inermis from Mount Gadut as N. bongso.

[2] Nepenthes inermis is endemic to a number peaks in the Barisan Mountains that line the western side of Sumatra.

It usually grows as an epiphyte in mossy forest, but also occurs terrestrially in stunted upper montane vegetation above 2,000 metres (6,600 ft).

It is so viscous that if a pitcher is held upside down, the fluid will pour out, forming an unbroken stream several metres in length.

[7][15] The fluid also acts as a lubricant, allowing prey items to easily slide down into the bottom of the pitcher cup.

The rainwater that accumulates in them is lost, but the extremely viscous fluid, together with the laterally appressed walls, ensure that the contents is retained.

[7][16] A similar trapping method may be employed by related species such as N. dubia, which also produce very viscous pitcher fluid.

[7][8] It has been suggested that glands on the lid secrete compounds that intoxicate visiting insects, such that they lose their footing and fall into the pitcher.

[14][16] Nepenthes inermis belongs to a group of closely related montane Sumatran species that includes N. dubia, N. flava,[17] N. jacquelineae, N. jamban,[18] N. talangensis, and N. tenuis.

[7] In 2001, Charles Clarke performed a cladistic analysis of the Nepenthes species of Sumatra and Peninsular Malaysia using 70 morphological characteristics of each taxon.

[7] N. aristolochioides N. tenuis N. dubia N. inermis In his description of the Bornean species N. campanulata, botanist Shigeo Kurata suggested that it may be closely related to N.

[20] While these two species are similar in general stature and pitcher morphology, they are not only separated by great geographical distance, but also occur in completely different habitats; N. campanulata is a lowland species endemic to limestone substrates,[21] whereas N. inermis usually grows as an epiphyte at elevations of 1,500–2,600 metres (4,900–8,500 ft).

[7][22] Nepenthes inermis is known to hybridise with N. talangensis on the upper slopes of Mount Talang, where the two species grow sympatrically.

In an article published in 1973 on the Nepenthes of Borneo, Singapore, and Sumatra,[19] Shigeo Kurata incorrectly identified specimens of this hybrid as belonging to N.

[7] In 1997, Matthew Jebb and Martin Cheek published their monograph "A skeletal revision of Nepenthes (Nepenthaceae)", in which they referred to N. dubia plant material from Mount Talang (Kurata s.n.

Clarke found that the type specimen of N. pyriformis, Kurata & Mikil 4230 NDC, matches the appearance of N. inermis × N. talangensis "in most respects".

[7][10] Folia mediocria sessilia, lamina spathulato-lanceolata, nervis longitudinalibus utrinque c. 3, basi caulis partem 1/2-1/3 amplectente, vagina 0 ; ascidia rosularum et inferiora ignota ; ascidia superiora parva, parte inferiore tubulosa v. paulum ventricosa, supra medium ampla infundibuliformia, ore expanso, peristomio fere 0, operculo angustissime cuneato, facie inferiore plana ; inflorescentia racemus parvus pedicellis inferioribus 4-8 mm longis, omnibus 1-floris ; indumentum iuventute tenue adpressum, denique in ascidiis sub ore et in inflorescentiis permanens.