Pur is fire in Greek. The vowel ypsilon used to be transcribed as 'y' but is usually transcribed as 'u' these days. At some point in Greek's phonological history, 'y' was pronounced /u/, but soon came to be pronounced as /ü/ and then /i/. If I could chance the Unicode Greek font in AWADtalk it might be clearer for those who already know Greek. Anywho, the root is cognate with the English word fire, from *pewor, *pur- 'fire'. Other fire roots include the one that Latin ignis and Sanskrit agni are related to, *egnis ~ *ognis 'fire'. In the end all these roots (here [cribbed from Pokorny dictionary] and in the A-H appendix [same source, but with some independent scholarship added]) are reconstructions. It's important to realize that there's no proof one way or another of if they existed, how they might have been pronounced, and what they meant. It's best to think of them as a shorthand, mnemonic device for comparing all the daughter languages' words.