carpo-, carp- (cerp-) +

(Latin: to pluck, to pick out, to gather, to select)

The change of Latin "a" to "e" is due to the Latin phonetic law according to which in the unaccented and closed radical syllable of the second element of compounds, original "a" becomes "e".

—Dr. Ernest Klein,
A Comprehensive Etymological Dictionary of the English Language.

Don't confuse this carpo- unit with other unrelated "carpo- units":

Greek carpo- meaning "fruit";

Greek carpo- meaning "wrist".