n R Difficult of mastery--a matter gen: e. g. impregnable--a fort: precipitous or perilous--a road: obscure, puzzling, perplexing--a subject, a passage.
n R Difficult of mastery--a matter gen: e. g. impregnable--a fort: precipitous or perilous--a road: obscure, puzzling, perplexing--a subject, a passage.