The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
pet-
DEFINITION:
Also pet- (oldest form *pet1-). To rush, fly. Variant *pte1-, contracted to *pt-. Derivatives include feather, compete, perpetual, ptomaine, symptom, and hippopotamus. 1. Suffixed form *pet-r-.feather, from Old English fether, feather, from Germanic *fethr, feather. 2.–petal, petition, petulant; appetite, compete, impetigo, impetuous, impetus, perpetual, repeat, from Latin petere, to go toward, seek. 3. Suffixed form *pet-n-.panache, pen1, penna, pennate, pennon, pin, pinna, pinnacle, pinnate, pinnati-, pinnule; empennage, from Latin penna,pinna, feather, wing. 4. Suffixed form *pet-ro- in compound *aku-petro- (see ku-). 5. Suffixed form *pet-yo-.propitious, from Latin propitius, favorable, gracious, originally a religious term meaning “falling or rushing forward,” hence “eager,” “well-disposed” (said of the gods; pr-, forward; see per1). 6. Suffixed zero-grade form *pt-ero-.–pter; acanthopterygian, aminopterin, apteryx, archaeopteryx, coleopteran, dipteral, mecopteran, orthopteran, peripteral, plecopteran, pteridology, pterygoid, sauropterygian, from Greek pteron, feather, wing, and pterux, wing. 7. Suffixed zero-grade form *pt-ilo-.coleoptile, from Greek ptilon, soft feathers, down, plume. 8. Suffixed variant form *pt-no-.stearoptene, from Greek ptnos, winged, flying. 9. Reduplicated form *pi-pt-.ptomaine, ptosis; asymptote, peripeteia, proptosis, symptom, from Greek piptein, to fall, with verbal adjective pttos (< *pt-to-), falling, fallen, and nominal derivatives ptsis (< *pt-ti-), a fall, and ptma (< *pt-m), a fall, fallen body, corpse. 10. O-grade form *pot-.hippopotamus, potamology, from Greek potamos “rushing water,” river (-amo-, Greek suffix). 11. Suffixed form *pet-tro-.talipot, from Sanskrit pattram, feather, leaf. (Pokorny 2. pet- 825.)
- (oldest form *pet
-. 
-. feather, from Old English fether, feather, from Germanic *fethr
, feather. 2. –petal, petition, petulant; appetite, compete, impetigo, impetuous, impetus, perpetual, repeat, from Latin petere, to go toward, seek. 3. Suffixed form *pet-n
), a fall, fallen body, corpse. 10. O-grade form *pot-. hippopotamus, potamology, from Greek potamos “rushing water,” river (-amo-, Greek suffix). 11. Suffixed form *pet-tro-. talipot, from Sanskrit pattram, feather, leaf. (Pokorny 2. pet- 825.) 
