Proto-IE: *peyǝ-, *pyV:-
Meaning: to grow fat; fat (milk)
Old Indian: páyate, pyā́yate `to swell, abound, increase, grow', ptc. pīna-, pínvati `to cause to swell'; pīpiváṁs- (f. pipyúṣī) `swelling, overflowing'; páyas n. `any fluid or juice, esp. milk, water, rain; semen virile'; péru- `swelling or causing to swell'; pī́vas- n. `fat', pī́van-, f. pī́varī `swelling, fat', pīvara- `fat, stout'; pitú- m. `juice, drink, food'
Avestan: (a)-pipyūšī `(keine) Milch in der Brust habend, (nicht) säugend', frapinoiti `macht schwellen, bringt zum Gedeihen', prapinvata `schwoll hin, floss hin zu-', payah- n. `Milch', paēman- n. `Muttermilch', pīvah- n. `Fett, Speck', pitu- m. `Saft, Trank, Nahrung, Speise'
Old Greek: pīmelǟ́ а. `Fett, Schmalz'; pīeró-, pīaró-; pī́ōn, f. pī́ei̯ra `fett, fruchtbar, reich', pī̂ar n. `Fett', piái̯nō `fett machen, mästen, bereichern'
Latin: opīmus, -a `fett, wohlgenahrt; fruchtbar; reich'
Russ. meaning: жиреть; жирный (о молоке)
References: WP II 73 f
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