From Longman Dictionary of Contemporary Englishassume a manner/air/expression etcassume a manner/air/expression etcformalPRETEND to behave in a way that does not show how you really feel, especially in order to seem more confident, happy etc than you are SYN put on Andy assumed an air of indifference whenever her name was mentioned. → assume
Examples from the Corpus
assume a manner/air/expression etc• Oliver assumed an expression of extreme penitence.• The only thing she could do was to assume an air of indifference.• No wonder the technique assumed an air of planet-wide importance. 