elbowboard

from The Century Dictionary.

• noun The board at the bottom of a window which forms the inner sill.
• noun In psychology, an apparatus for determining the just noticeable (passive) motion of the forearm, that is. the just noticeable change of articular sensation at the elbow. Also called arm-board.

from the GNU version of the Collaborative International Dictionary of English.

• noun The base of a window casing, on which the elbows may rest.