The project manager was held accountable when the delivery was significantly delayed.

accountable 是形容词，由动词 account（解释、说明）加后缀 -able（能够...的）构成，字面意思是"能够被要求解释的"，引申为"负有责任的、应作解释的"。 在句中表示项目经理需要为交付延迟承担相应责任。