bent themselves nearly double

[Tinkerbell]

And then the three bent themselves nearly double and crept stealthily down the stairway with cocked pistols in their hands and glittering knives between their teeth, leaving Martha trembling with fear and too horrified to even cry for help. :roll:

 

[magimagicE]

If you fold a body at the waist, then that body would be bent double.

"bent themselves nearly double", simply means that they were in a low-down, crouching attack posture, slighty bent at the waist.

There's a famous poem by Wilfred Owen called Dulce et Decorum Est that makes good use of the imagery of "bent double".

 

[Tinkerbell]

Thank you magimagicE :-D