a question, person, thing, circumstance, that is puzzling

mystery

something of which the cause or origin is impossible to explain or understand

puzzle, secret

stubborn

pig-headed, stubborn

a question, statement or description, intended to make a person think hard in order to know the answer or meaning; to make many holes in a person, thing

brain-teaser

stiff; that cannot be bent; firm; strict; not changing

stiff, strict, severe