pre-noun adjectival
meaningless; senseless; absurd
Literary pre-noun adjectival (rentaishi) meaning something is without reason, groundless, or pointless. Often used in formal or written contexts.
由なき噂に惑わされてはいけない。
Don't be misled by groundless rumors.
彼の怒りは由なきものだった。
His anger was senseless.
無意味 is a common noun/adjectival noun meaning 'meaningless', while 由なき is a literary pre-noun adjectival with a nuance of 'groundless' or 'without reason'.
理不尽 means 'unreasonable' or 'irrational', often implying unfairness, whereas 由なき focuses on the lack of basis or justification.
Composed of 由 (reason, cause) + なき (classical negative form of ある, 'to be'), literally 'without reason'. The form is a fossilized classical construction used as a pre-noun adjectival.