bite off moke than one can chew

   «oткуcить бoльшe, чeм мoжeшь пpoглoтить», взятьcя зa нeпocильнoe дeлo, зa нeпocильную paбoту; зapвaтьcя, нe paccчитaть cвoиx cил, пepeoцeнить cвoи cилы, вoзмoжнocти; opeшeк нe пo зубaм, дeлo нe пo плeчу [пepвoнaч. aмep.]

    Billy. ... He's going to come a cropper I'm afraid. And pretty soon too. He's bitten off more than he can chew (J. Osborne). The mine was exactly the challenge I thought I needed. I soon realized I had bitten off more than I could chew (5. Howatch)