Green’s Dictionary of Slang

1. (also heel up) to arm oneself with a firearm.

1873		J. Miller Life Amongst the Modocs 301: This was his signal to ‘heel’ himself and come upon the ground [DA].
1876		‘Dan de Quille’ Big Bonanza (1947) 276: His man had gone off to ‘heel himself,’ and there would soon be trouble.
1931		(con. 1920s) C.W. Willemse Behind The Green Lights 330: Come on right over and be heeled. We’re ready to roll.
1947		W.A. Chalfant Gold, Guns and Ghost Towns 72: Vance posed as one of the original bad men, and abused Carberry until the latter told him to wait until he could ‘heel himself’ [DA].
1950		Goldin et al. DAUL 94/1: Heel up. To arm oneself, especially with a gun or a knife.

2. (US) to lend money.

1873		J. O’Connor Wanderings of a Vagabond 360: He’s got an industrious gal, an’ she allers heels him to a stake in ther mornin’.
1893		St Louis Post-Dispatch (MO) 3 Dec. 17/7: To ‘stake’ and to ‘heel’ mean to lend .