Douglas Harper's Etymology Dictionary
Wiktionary
shadow boxing
n. (context boxing English) The action of practice moves where the opponent is the boxer's own shadow. vb. (present participle of shadow box English)
Usage examples of "shadow boxing".
The program, which had consisted in part of shadow boxing and bag punching by Young Brophy, was to terminate with three rounds with Jimmy.
The chief in the velours hat gave himself an air of boredom, did a little shadow boxing, which made the long sleeves of his raincoat slide up to his elbows, and held up his wristwatch in the moonlight.
Gregg has failed to make contact with anybody and his flailing resembles shadow boxing.