Apparently, the phrase ‘a Sunday punch’ means a killer punch, a knockout, either in boxing terms or metaphorically in other areas like politics. A normal person would look at that phrase and try to think of a reasonable and sane way of explaining it in English, which isn’t difficult. Most boxing matches probably occurred on Saturday night, so a Sunday punch is surely one that puts you out of action until the following day. Another common expression, ‘to knock someone into the middle of next week’, uses the same metaphor.
However, Cassidy was not a normal person and he had no interest in sane and reasonable explanations. He immediately took his Irish dictionary in hand and found what he thought was a suitable word, sonnda.
‘Sonnda, al. sonnta, adj., powerful, strong, courageous, bold (punch). (Ó Dónaill, 1134; Dineen, 1088.)’
As usual in Cassidy’s ‘research’, this is not a real quotation. Sonnda derives from the word sonn, which means a stake or post, so sonnda is a very old-fashioned, literary word for powerful, steadfast, and would be used of a castle or a fortification, not a blow. To confuse matters, Dinneen gives it as an alternative spelling of sonnta, which means forceful, pushy or cheeky. In other words, Cassidy is mixing the entries for two distinct terms. Moreover, none of the meanings attached to the words sonnda or sonnta would lead you to believe that they would ever be applied to blows or punches. There are lots of adjectives which would be used in this way with the word for blow (buille) – buille trom, buille treascrach, buille cumhachtach, buille láidir, buille tolgach.
Sonnda isn’t one of these adjectives, so Cassidy’s claim is just as daft as the rest of this absurd book.