whistle in the dark

whistle in the dark

1.be whistling in the dark (be confident about something although you have no good reason to be confident) — не поддаваться страху; xpабриться, хорохориться;

Example 1:He wasn't very sure of his chances at the time - he was really just whistling in the dark, hoping to get some support. (CDAmI)

2. be whistling in the dark (to guess aimlessly; to speculate as to a fact)

Example 1: Am I close, or am I just whistling in the dark? (McGraw-Hill Dict)
Example 2: She was whistling in the dark. She has no idea of what's going on. (McGraw-Hill Dict)

also
whistling through the graveyard - to show bold face even when scared