na-adjective
Describes a blade or cutting edge that has lost its sharpness. Often used metaphorically for a person's dullness or lack of sharpness.
この包丁はなまくらで、トマトも切れない。
This kitchen knife is so blunt it can't even cut a tomato.
Used to criticize a person as lacking spirit, energy, or courage; implies a dull, sluggish character. Often derogatory.
あんななまくらな奴に仕事を任せられない。
I can't leave work to such a good-for-nothing guy.
なまくらなことを言うな。
Don't say such cowardly things.
にぶい is the standard adjective for 'dull' or 'blunt' (both physical and mental), while なまくら is more colloquial and often carries a derogatory tone for people.
怠け者 means 'lazy person' and is a noun; なまくら is an adjectival noun that can describe a person's character more broadly, including cowardice.
The origin is uncertain. Possibly from 鈍 (なま) meaning 'dull' and an emphatic suffix くら, but the exact derivation is unclear.