also: きにすんな
expression, interjection
never mind!; don't worry about it
Casual imperative used to tell someone not to be concerned or bothered. The contracted form 気にすんな is even more informal.
失敗しても気にするな。
Don't worry about it even if you fail.
気にすんなって。大丈夫だから。
I said don't worry. It's fine.
Negative imperative form of 気にする (to mind, to worry). The contracted form 気にすんな comes from casual speech contraction of するな to すんな.