They inhabited montane dry forests dominated by ʻakoko (Euphorbia species) and ōpuhe (Touchardia sandwicensis).

The Lānaʻi hookbill was a plump, medium-sized bird with greenish olive upperparts and pale whitish yellow underparts.

The hookbill's distinguishing characteristic was its heavy, parrotlike bill, which had the mandibles hooking sharply towards each other, leaving a gap between them when the beak was closed.

The combination of habitat destruction and the introduction of feral cats and rats are thought to have led to the Lānaʻi hookbill's extinction.

[2][3] In 1919 Robert Cyril Layton Perkins described the species as Dysmorodrepanis munroi based upon this specimen, placing the hookbill in a new, monotypic genus.

[2] The genus name is derived from the Ancient Greek words dusmoros "ill-fated," and drepanis to identify the species as a Hawaiian honeycreeper.

[4] However, other taxonomists challenged the validity of the species as early as 1939, noting that the Lānaʻi hookbill was only known from one specimen and arguing that it was merely an aberrant and partially albino female ʻōʻū.

[8] Due to the subdued colors of the sole specimen, it is believed that it was a female, suggesting that the male would have had a brighter plumage, especially in the superciliary line.

[8] The eyes, which were large for a bird of the hookbill's size, were dark brown and the muscular legs were gray with yellow toepads.

[8] The bird's only known vocalization was an inconspicuous chirp; however, all other Hawaiian honeycreepers are excellent vocalists that demonstrate an array of sounds, and therefore the hookbill likely had a broader, unrecorded repertoire.

[7] All recorded sightings of the species were made from the southwestern end of Lānaʻi's forests, which included the Kaiholena Valley and Waiakeakua.

[6] It is considered likely that the hookbill additionally ate ʻakoko fruits due to their similarity in size and shape to those of the ōpuhe.

[15] However, the other Hawaiian honeycreepers are remarkably uniform in their breeding behavior, and it is therefore likely that the hookbills also bred from early winter through the end of summer in August, with pair bonding being completed by January or February.

This conversion reduced the area of the hookbill's potential habitat, and is believed to be the biggest contributor to the species' extinction.