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Louis C.K. did NOT tread lightly in his first stand-up gig since admitting his sexual misconduct, because he reportedly cracked a joke about rape whistles.

Remember, C.K. copped to dropping his pants and masturbating in front of women ... and even coercing them into keeping silent about it. So, at least 2 female audience members at the Comedy Cellar in NYC were shocked to hear him start a bit about the phrase "clean as a whistle."

The women told Vulture C.K. worked up to a joke about rape whistles NOT being clean, and they say it was very well received -- "people were laughing, and I was just sitting there like, oh my f**k."

They were uncomfortable and depressed that C.K.'s joke had killed ... especially considering the things he's admitted doing.

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