anhedonia n.

"absence of pain, incapacity of feeling pain in a part, though tactile sense is preserved," 1706, medical Latin, from Greek analgesia "want of feeling, insensibility," from analgetos "without pain, insensible to pain" (also "unfeeling, ruthless"), from an- "not" (see an- (1)) + algein "to feel pain" (see -algia). An alternative form is analgia.

privative prefix, from Greek an-, "not, without" (from PIE root *ne- "not"). The Greek prefix is a fuller form of the one represented in English by a- (3).