From Longman Dictionary of Contemporary Englishbe whistling in the darkbe whistling in the darkinformalBRAVE to be trying to show that you are brave when you are afraid, or that you know about something when you do not Does he know what he’s talking about or is he just whistling in the dark? → whistle
Examples from the Corpus
be whistling in the dark• I'm just whistling in the dark on this one - I could really use some help. 