打了棍半 One And A Half Strokes
大聖道：「不許說謊，你要砍三刀，只砍了我兩刀。教我打一棍，若打了棍半，就不姓孫。」老魔道：「正是，正是。」大圣道：「不许说谎，你要砍三刀，只砍了我两刀。教我打一棍，若打了棍半，就不姓孙。」老魔道：「正是，正是。」
逐句解 Line by line
The Great Sage said, "No lying now. You were to cut three times and you have cut me only twice."
He is keeping the books. Three cuts were agreed, two have been spent, and he wants the ledger acknowledged before the terms change.
"You bid me strike one blow with the cudgel. If I strike a blow and a half, my surname is not Sun."
教 is jiào, to bid or make somebody do. 棍半, a blow and a half, is the joke — he is promising precision in a unit nobody measures in. Swearing on the surname is the standard vernacular oath, and coming from a monkey given his surname by a patriarch it carries more weight than it looks.
The old fiend said, "Just so, just so."
Doubled agreement, exactly as he doubled it when his brother proposed the bottle. This chief settles everything twice and has been wrong on both occasions.
Wukong's counting is not pedantry, it is the whole of his advantage. He has established that a cut is owed him, that the swap was the demon's idea, and that the terms are one blow — all before he lifts the cudgel. The fight has been won in the negotiation.
The oath on the surname is a small piece of self-definition. Sun is not a name he was born with; the patriarch Subodhi gave it to him in the first chapters, reasoning from the character for an ape. Staking it on a promise of accuracy is the nearest this creature comes to a formal guarantee.