(From A Terrible Kindness by Jo Browning Wroe, Part II Cambridge Choir, chapter 12 September 1957)
- (The boy chorister saying goodbye to his mum)
'I'm so proud of you,' Evelyn (his mum) is saying, the tide in her eyes still high.'And so would your father be. You know that, don't you?'
'Yes.' He puts his hand on her shoulder because he can't bear her to look sad. She stands up so quickly, his hand slaps down onto the grey flannel of his new shorts.
'Don't start being all nice to me, William Lavery, you'll set me off.' She smiles down at him, blinking.'I'm allowed to come and see how you're getting on in six weeks. . . ..'
I know what "set somebody off laughing/crying" etc. means, but the ing-form is missing there. What does "You'll set me off" (without the ing-form) mean? Is there a synonym? Does it mean something like "You'll put me out of countenance"?