Don't carelessly let secrets slip!

Literal

Carelessly secrets [object-を] leak [must not-ちゃだめだよ].

不用意に = 'carelessly/inadvertently.' 洩らす = 'to leak/divulge.' ~ちゃだめだ is a casual prohibition: contraction of ~てはだめだ ('mustn't'). よ adds assertion.