О-101

КРЕПКИЙ (ТВЁРДЫЙ) ОРЕШЕК (ОРЕХ) coll NP sing only)

1. a person who is unbending, difficult to deal with, convince, subjugate etc

tough cookie (nut, customer).

«Напрасно вы упорствуете... Ответьте нам на один вопрос, и мы отпустим вас в камеру отдыхать. Так всё-таки кто же вас заслал в деревню Красное?» - «Кому надо, тот знает», - сказал Чонкин, отдуваясь. Словно кувалдой дали ему в подбородок... «Крепкий орешек», - потирая ушибленную руку, задумчиво сказал полковник (Войнович 4). "There's no point in resisting... .Answer one question and we'll let you go rest in your cell. Who sent you to the village of Krasnoye?" "That's known by who's supposed to know," said Chonkin, panting. The next punch hit his chin like a sledgehammer.... "A tough nut," said the colonel pensively, rubbing his bruised hand (4a).

2. (of a matter, concept) difficult to understand, (of a problem) difficult to solve, (of an undertaking) difficult to accomplish, (of a fortress, city etc) difficult to capture etc

tough (hard) nut to crack.