countersign
v[Tn]
sign (a document, etc already signed by another person 副署; 会签)
a cheque countersigned on the back 背书支票.
countersign n secret word which must be spoken to a guard, etc before one is allowed to pass; password 回令(回应警卫人员等的暗语以便通行)
give the countersign 应答回令.