Adj. 1. gobsmacked - utterly astounded colloquialism - a colloquial expression; characteristic of spoken or written communication that seeks to imitate informal speech Britain]], UK, United Kingdom, United Kingdom of Great Britain and Northern Ireland, Britain, GB - a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland surprised - taken unawares or suddenly and feeling wonder or astonishment; "surprised by her student's ingenuity"; "surprised that he remembered my name"; "a surprised expression"