Well, it's steam driven
I heard that the term originally referred to Hueys in Vietnam, and one possibility is:
"Bell originally was in the business of building Steam Locomotives. Helicopters came at a later date. In SEA some of the Heuys had amour plating on the lower section, also referred to as "Chicken Plates". Hence the name Steam Chicken. That and the fact that there was "steam" vapour (vapor for TQN) coming out the breather."
But this is from a senior Canadian pilot:
"UH-1 = Huey = cartoon Character "Baby Huey" ( Was a large, tubby chicken ) + 205 kinda sounds like a steam engine from far away and breather pumps puffs of smoke outa the exhaust = "Steam Chicken."
And another:
"Steam chicken refers to the civilan 212 and comes from the engineering fraternity."
Take your pick!
Phil