Scallywags are non-union members, who are given sanction (by the International Longshoremen’s Association) to work when not enough card-carrying longshoremen are available to handle the workload.
Source:
"On the Waterfront"
By Phil Primack, Boston Magazine, March 2006
Last Monday, a ship arrived in port carrying more cargo than the union longshoremen could unload within the required time period, so scallywags were allowed to work with union members on that job.