"sling arms" (not in OED2) -- [1749], 1824

[Amy West]

And to add to Dan's confusion, I've found "slung" in relation to a lance
in _A new system of broad and small sword exercise : comprising the
broad sword exercise for cavalry and the small sword cut and thrust
practice for infantry. to which are added, instructions in horsemanship_
by Thoms Stephens, (Philadelphia : I. R. & A. H. Diller, 1843.), :

BUT it's because the lance has a sling:

Dismount. . .

The lance, in all movements, is to be at the "Carry" or "Trail," except
on marches, when it is to be slung on the right or left arm; or when the
men sit at ease, then the lance is to rest in the hollow of the left
shoulder, the right hand upon it, extended down on the shaft. . . .

Carry Lance  -- One -- The lnace is to rest with the butt-end in the
bucket, on the right stirrup, and to be kept perpindicular by the right
hand, which is to be in a line with the shoulder, the thumb in the sling.

(p. 87)
(via HathiTrust)

---Amy

------------------------------------------------------------
The American Dialect Society - http://www.americandialect.org