Clauson (1972)
An etimological dictionary of pre-thirteenth-century Turkish
p. 687
köp-
‘to swell, foam, boil over’. Not noted before xiv, but see köpük; survives in NE, SE, and NC Kır. Kom. xiv ‘to swell’ köb- CCG; Gr.: Kıp. xv fâra ‘to boil over’ (taş-/) köp- (-b-) Tuh. 28b. 1: Osm. xvi köp- ‘to swell’; in one text TTS II 658.