b.to offer or give a knee, to act as second in a pugilistic encounter, it being customary for a second to give a principal the support of his knee between the rounds.
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society > leisure > sport > types of sport or game > fighting sports > boxing > box [verb (intransitive)] > act as second
to offer or give a knee1847
1847 W. M. ThackerayVanity Fair (1848) v. 37 Everybody was anxious to have the honour of offering the conqueror a knee.
1857 T. HughesTom Brown's School Daysii. v. 321 Tom, with..Martin to give him a knee, steps out on to the turf.