[mid-18C tailors’ jargon dung, a tailor who accepts the master’s terms without argument, or who works when his fellows are striking; the dung is ‘soft’ (and disgusting), while the union man, the flint, is ‘hard’ (and admirable)]
[late 18C–19C] a workman who accepts less than union wages; thus a strike-breaker.
SE in slang uses
[1960s+] (Aus.) a derog. term for a male homosexual.