Boxing. the most powerful and effective punch of a boxer, especially the punch used in trying to gain a knockout.
anything, as an armed force, line of argument, etc., capable of inflicting a powerful blow on a hostile or opposing person or thing.
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British Dictionary definitions for Sunday punch
boxing a heavy blow intended to knock out one's opponent
any manoeuvre or action intended to crush an opponent
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012