Also petə-, Proto-Indo-European root meaning "to rush, to fly."
It forms all or part of: accipiter; appetence; appetite; apterous; apteryx; archaeopteryx; asymptote; centripetal; Coleoptera; compete; competent; eurypterid; feather; helicopter; hippopotamus; Hymenoptera; impetigo; impetuous; impetus; iopterous; Lepidoptera; ornithopter; panache; panne; pen (n.1) "writing implement;" pennon; peripeteia; perpetual; perpetuity; petition; petulance; petulant; pin; pinion; pinnacle; pinnate; pinniped; potamo-; potamology; propitiation; propitious; ptero-; pterodactyl; ptomaine; ptosis; repeat; symptom.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pattram "wing, feather, leaf," patara- "flying, fleeting;" Hittite pittar "wing;" Greek piptein "to fall," potamos "river, rushing water," pteron, pteryx "feather, wing," ptilon "soft feathers, down, plume;" Latin petere "to attack, assail; seek, strive after; ask for, beg; demand, require," penna "feather, wing;" Old Norse fjöðr, Old English feðer "feather;" Old Church Slavonic pero "feather;" Old Welsh eterin "bird."
early 15c., from Old French vehement, veement "impetuous, ardent" (12c.), from Latin vehementem (nominative vehemens) "impetuous, eager, violent, furious, ardent, carried away," perhaps [Barnhart] from a lost present middle participle of vehere "to carry" (from PIE root *wegh- "to go, move, transport in a vehicle"). The other theory is that it represents vehe- "lacking, wanting" + mens "mind." Related: Vehemently.
"confusedly; in an impetuous rush; with indiscriminate violence, energy, or eagerness," 1570s, from French pêle-mêle, from Old French pesle mesle (12c.), apparently a jingling rhyme on the second element, which is from the stem of the verb mesler "to mix, mingle" (see meddle). The phonetic form pelly melly is attested in English from mid-15c.