bun

to be; being; exist; become; happen

From Avestan bun or bû " to be, to become, will become".

*bheuə- or *bheu-, Proto-Indo-European root meaning "to be, exist, grow",

Sanskrit bhavah,
Old High German bim "I am”,
Old Church Slavonic byti "be",
Greek phu- "become",
Old Irish bi'u "I am",
Lithuanian būti "to be",
Russian byt.