Shuck-shuck is when the US player uses a fleet of transports to move land units (mostly infantry) from Eastern Canada (and Western C, in most rule variations) to Europe, usually Finland. The idea is to get a constant stream of US land units into Europe where they can fortify Karelia, shift to defend Russia, launch minimal-force attacks into Ukraine, or whatever is needed.
After a few turns, US can have 5 trans, and build say, 8 inf, 1 arm per turn to load them. These can be spread between East US and West US however the player wants (again, assuming rules allow pickup from W Canada). On the non-com phase, move the land units to East or West Canada respectively. On the next turn, these units will be picked up by transports in the UK SZ, from E Canada SZ, and returned to the UK SZ where they can be unloaded in Finland, or alternatively in WE, if an attack is viable. Another alternative is to land in Algeria, or Spain, depending on strategy, from the Spain SZ.
I think the poster above accurately pointed out where the term comes from (this guy Don, who wrote some essay about A&A strategy - http://donsessays.freeservers.com/). He said the process makes the sound "shuck shuck", whatever that means.