From Longman Dictionary of Contemporary EnglishLord (only) knowsLord (only) knowsspoken a) used when you do not know the answer to somethingLord knows how/who/where etc Lord knows how old she is now. b) used to emphasize that something is true Lord only knows I tried my best to get this right. → Lord
Examples from the CorpusLord (only) knows• Even Protestants - and the good Lord knows how mistaken they are - have something to hold on to.• We need heroes. Lord knows, kids need them today.• I do mean it! Lord knows I read plenty of nonfiction books.• When I was a young houseman I - well, enjoyed a hectic love-life. Lord knows how I managed it.• The Lord knows I did my best.• The Lord knows why she hung them where she would see them every time she looked up.• The Lord only knows what will happen in the regular season and playoffs next year.• You realize there was a major pile-up? Lord knows how many vehicles - all of them following us, I suspect.