A polo player, or a period of play in polo.
From 'chukka' combined with the agent suffix '-er,' creating 'chukker' to mean one who plays during a chukka, or in a broader sense, a polo player.
The '-er' suffix is one of English's most productive word-making tools—you can add it to almost any verb or noun to mean 'one who does that'—so 'chukker' literally means 'one who plays chukkas!'
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