peth₂-
“to fly, to fall”fly, rush, fall
Root for flying/falling, yielding Latin petere (to seek), Greek pteron (wing), English feather.
Discussion
The Proto-Indo-European root *peth₂- meant "to fly" or "to rush," with a secondary sense of "to fall." In Latin, petere meant "to seek, to aim at, to rush toward": petition (a seeking), compete (to seek together), appetite (a seeking toward), repeat (to seek again), and perpetual (seeking through to the end). The Greek reflexes preserved the aeronautic meaning more faithfully. Pteron, "wing" or "feather," appears in pterodactyl (wing-finger) and helicopter, which splits as helico-pter (spiral-wing), not heli-copter. The o-grade pótamos, "river" (that which rushes), gave hippopotamus. The Germanic branch underwent Grimm's Law, shifting *p to *f, producing English feather from Proto-Germanic *feþrō, a direct cognate of Greek pteron. The same shift gave English pen, borrowed from Latin penna (earlier pesna), itself from *pet-na, "that which flies" — making pen and feather doublets from the same root. Sanskrit pátati, "he flies, he falls," preserves both senses in a single verb, suggesting that the Proto-Indo-European speakers themselves had not yet fully distinguished the two trajectories. The root captures a primordial unity of upward and downward motion, of aspiration and descent, the wing and the crash.
Notes
Pokorny 825-826. English feather, pen, petition, compete, repeat.