This, according to a chef I know, is how they would describe a chicken leg or thigh that had been cut at the knuckle.

http://www.ehow.com/how_2274769_trim-chicken-gourmet-chicken...

--------------------------------------------------
Note added at 1 day18 hrs (2011-05-28 07:56:42 GMT)
--------------------------------------------------

The term describes removing waste parts, or parts not needed, and is not strictly limited to describe cutting away excess fat or skin, although it is used for this sometimes. The term was actually confirmed to me by a professional chef, who says she would use this term to describe cutting away any unused part of the chicken.
This web page shows its usage in this context:
"I trim the thighs to a uniform shape, I want them all to look the same. ON THE KNUCKLE END I JUST USE A KNIFE TO TRIM IT OUT." http://tvwbb.com/eve/forums/a/tpc/f/7980069052/m/1720059365
"Take the thigh bones out of the legs and TRIM OFF THE KNUCKLE OF THE DRUMSTICK (you can leave the bone in, but it won t leave you much room for the stuffing)." http://www.bbc.co.uk/food/recipes/wholeroastchickenpot_13472
"...to cut up chicken, remove ribs, TRIM WISHBONE and tendon OFF..." http://www.cookingforengineers.com/article/123/Boning-Chicke...


--------------------------------------------------
Note added at 1 day18 hrs (2011-05-28 07:57:52 GMT)
--------------------------------------------------

Re note above:
The idea of using the term "trim" is because the part being cut away in your recipe, is not actually needed because it is not part of the eaten food.