before vowels phren-, word-forming element meaning "mind," also, in medical use, "diaphragm, muscle which parts the abdomen from the thorax;" from Greek phrēn, phrenos "the mind, spirit," also "the midriff, diaphragm," also (in plural, phrenes) "the parts around the heart, the breast," and hence "wits, sense, sanity, mind" on the notion of the breast or heart as the seat of thoughts and passions.

The word is of uncertain origin; Watkins has it under a proposed PIE root ‌‌*gwhren- "to think." Beekes finds the connection with phrassein "to fence or hedge in" "semantically attractive," but there are phonetic difficulties, and he finds "quite feasible" a relationship with phrazomai "to think, consider" (later phrazein; see phrase (n.)), itself an isolated word.

word-forming element in modern science meaning "seaweed, algae," from Latinized form of Greek phykos "seaweed, sea wrack," also "rouge, red make-up made from seaweed;" Beekes writes that it is a loan-word from Semitic and compares Hebrew pūk "eye-rouge." "The meaning 'make-up' is therefore primary for [phykos], too; hence 'seaweed'." Compare fucus, which is probably a Latin borrowing of the Greek word.

before vowels phyll-, word-forming element meaning "leaf," from Greek phyllon "a leaf" (from PIE *bhol-yo- "leaf," suffixed form of root *bhel- (3) "to thrive, bloom").

before vowels phyl-, modern word-forming element, mostly in the sciences, often meaning "phylum," from Greek phylon, phylē "a tribe," also a political subdivision in ancient Athens, from base of phyein "to bring forth, produce, make to grow," whence also physis "nature" (from PIE root *bheue- "to be, exist, grow").

word-forming element meaning "physical, physically; natural," from Latinized form of Greek physikos "natural, physical, pertaining to nature" (from PIE root *bheue- "to be, exist, grow"). Compare physic.

word-forming element meaning "nature, natural, physical," from Greek physios "nature" (from PIE root *bheue- "to be, exist, grow").

word-forming element used in making names for very small units of measure, 1915 (formally adopted as a scientific prefix meaning "one trillionth" by the International System of Units, 1960), from Spanish pico "a little over, a small balance," literally "sharp point, beak," a word of Celtic origin (compare Gaulish beccus "beak").

word-forming element meaning "pressure," from Greek piezein "to press tight, squeeze," from PIE *pisedyo- "to sit upon" (source also of Sanskrit pidayati "presses, oppresses"), from *pi "on," short for *epi (see epi-) + root *sed- (1) "to sit." First in piezometer (1820), an instrument for ascertaining or testing pressure. It was in common use in word formation from c. 1900.

before vowels pithec-, word-forming element meaning "pertaining to an ape or monkey," from Greek pithēkos "ape," which Beekes calls "A loanword from an unknown language."