Green’s Dictionary of Slang

1. (US) a soldier, whose diet is mainly pork [from sense 2].

1850		L.W. Garrard Wah-to-yah and the Taos Trail x 155: Numerous were the curses showered on the ‘Neds,’* by the mountain men of our party. [*Note:] Among many farmers, pork is familiarly called ‘Ned,’ and as pork forms a principal portion of the government rations, the United States employees were so termed, by the mountain men, in derision.
1953		Randolph & Wilson Down in the Holler 97: Soldiers were called neds also, because they fed largely upon pork.

2. (US) salt pork or bacon, usu. constructed with old.

1850		see sense 1.
1913		H. Kephart Our Southern Highlanders (1922) 75: ‘Bill, hand me some Old Ned from that suggin o’ mine.’ [...] I learned that ‘Old Ned’ is merely slang for fat pork.
1953		Randolph & Wilson Down in the Holler 97: The word ned means bacon in the Ozarks.

3. (US black) a black person who curries favour with white society [? the image of such a figure being effectively a pig n. (1a)].

1964

PADS XXXIX 29: Most of the terms [of abuse] collected exlusively from Negro informants are rare with the Causasian social, dialects of Chicago, e.g., Cuff, ned, blue, [...] and black fay.