Douglas Harper's Etymology Dictionary
phreno-
before vowels phren-, word-forming element meaning "mind," also "diaphragm," from Greek phreno-, comb. form of phrene properly "diaphragm, muscle which parts the abdomen from the thorax;" in Homer extended to "parts around the heart, breast," hence "heart" (as a seat of passions), "mind, seat of thoughts, wits, senses," of unknown origin.
Wiktionary
phreno-
pre. 1 Used to form terms related to the mind. 2 Used to form terms related to the diaphragm.