Green’s Dictionary of Slang

1. a hard blow.

1899

C. Rook Hooligan Nights 127: I got a flattener on my razzo that pretty nigh laid me out.

2. in fig. use, something defeating.

1849		John O’Groat’s Jrnl 23 Feb. 2/3: Mr S. comes down, as Jonathon would say, with a ‘flattener’ when he tells us [etc.].
1892		J. Kirkland Story of Chicago I 141: We have had a flattener pass over the face of our prospects. The lighthouse that, the day before yesterday, stood in all its glory, the pride of this wonderful village, is now ‘doused’.
1923		J. Bowes Jackaroos 186: By George, this is a flattener! This’ll be another knock at the boss.