Green’s Dictionary of Slang

(Anglo-Ind.) silent, quiet.

1877		A. Allardyce City of Sunshine II 8: ‘Many men are not so soft as they look, and many who bluster like lions in the bazaar are chup (silent) enough in the zenana.
1897		B.M. Croker Mr Jervis 253: ‘What’s the matter, eh? You seem rather choop’.
1933		E. Thompson Indian Day (1940) 9: The coolies were paid. As they looked at the coins in their palms, any discussion was foreclosed by an imperious wave of the hand, and an order to jao. ‘And jao jeldily, and be choop about it.’.