'phenom' originates from English as a shortening of 'phenomenon,' ultimately from Greek 'phainomenon,' where 'phainesthai' meant 'to appear' or 'to show.'

'phenom' developed in modern English as an informal clipped form of 'phenomenon' (which came into English via Latin and Middle French from Greek 'phainomenon').

Initially related to something that 'appears' or is observable; over time it came to mean 'an outstanding person or remarkable thing' in informal modern usage.