Green’s Dictionary of Slang

1. an informer.

1830

Lytton Paul Clifford III 239: ‘If he does not come to be scragged,’ says I, ‘it vill only be because he’ll turn a rusty, and scrag one of his pals!’.

2. a fight, a skirmish.

1835		R.M. Bird Hawks of Hawk-Hollow II 245: Neversomever, I’ll try for a spell ag’in, and the next’ll be a right-down rusty! [DA].
1837		R.M. Bird Nick of the Woods I 132: I hoped you would have been for taking a brush with us; and it war my idea to send a messenger after your party, in the hopes your men would join us in the rusty.