Green’s Dictionary of Slang

in difficulties.

1914		Bulletin (Sydney) 30 July 11/4: [cartoon caption] Bunkered. / She: ‘When we first met you thought I was fast.’ / He: ‘Oh, no, I didn’t. I only hoped you were!’.
1929		Kipling ‘Satisfaction of a Gentleman’ in Complete Stalky & Co. (1987) 257: Stalky plugged him – bending – on the edge of The Pit, and he fell into it, cursing Stalky for all he was worth. The Colonel was bunkered at the bottom.