Is thenar a word in the Scrabble Dictionary

Yes, thenar is playable

...and is worth 9 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Thenar" Definition

noun

The fleshy mass on the palm of the hand at the base of the thumb.

adjective

Of or relating to the thenar.
Of, relating to, involving, or constituting the ball of the thumb or the intrinsic musculature of the thumb.