Exact adjective of "conundrum"

Solution 1:

> Perplexing.

puzzling, complex, confusing, complicated, involved, hard, taxing, difficult, strange, weird, mysterious, baffling, bewildering, intricate, enigmatic, mystifying, inexplicable, thorny, paradoxical, unaccountable, knotty, labyrinthine

Solution 2:

Inscrutable is an extremely close synonym for the "difficult problem" definition.

Solution 3:

Puzzling. A conundrum is a puzzle or difficult problem, often a riddle. The word is pseudo-Latin; it was coined at Oxford in the 1590's according to OEtD. By 1790 its meaning had evolved to more or less what it is today.

Solution 4:

I'm after the "difficult problem" (but not the "pun") synonymity. It's used to describe a word from an ancient text which is seemingly not in the dictionaries; hard to find information about.

IMO, obscure, arcane or esoteric might be a better fit than "conundrous".