'To die in harness' definitions:

Definition of 'To die in harness'

From: GCIDE

Harness \Har"ness\ (-n[e^]s), n. [OE. harneis, harnes, OF. harneis, F. harnais, harnois; of Celtic origin; cf. Armor. harnez old iron, armor, W. haiarn iron, Armor. houarn, Ir. iarann, Gael. iarunn. Cf. Iron.]
1. Originally, the complete dress, especially in a military sense, of a man or a horse; hence, in general, armor. [1913 Webster]
At least we'll die with harness on our back. --Shak. [1913 Webster]
2. The equipment of a draught or carriage horse, for drawing a wagon, coach, chaise, etc.; gear; tackling. [1913 Webster]
3. The part of a loom comprising the heddles, with their means of support and motion, by which the threads of the warp are alternately raised and depressed for the passage of the shuttle. [1913 Webster]
To die in harness, to die with armor on; hence, colloquially, to die while actively engaged in work or duty. [1913 Webster]