osso duro de roer

== Portuguese == === Etymology === Literally, “hard bone to gnaw”. === Noun === osso duro de roer m (plural ossos duros de roer, metaphonic) (idiomatic) hard nut to crack (problem that is challenging to solve) (idiomatic) hard nut to crack (situation, person, group, etc. which is difficult to overcome or deal with) (idiomatic) a brave or hardy person or organisation === Further reading === “osso duro de roer”, in Dicionário Priberam da Língua Portuguesa (in Portuguese), Lisbon: Priberam, 2008–2026