godan verb (-ru), intransitive verb
keep completely silent; clam up; refuse to say anything
Emphatic verb meaning to stay stubbornly or persistently silent, often with a nuance of sullenness or defiance. The suffix こくる adds a pejorative or intensifying sense.
彼は何を聞かれても黙りこくっていた。
No matter what he was asked, he kept completely silent.
会議中、彼女はずっと
During the meeting, she clammed up the whole time and didn't say a single word.