A skeleton in the cupboard
Definition 1 of 1
Pro Tip 1/3Cultural Sensitivity
Be aware of cultural differences when discussing skeletons in the cupboard, as some topics may be more sensitive in certain cultures. 
He hesitated to share the skeleton in his cupboard while working abroad, understanding the cultural sensitivity of his issue.
Pro Tip 2/3Personal Topics
The phrase often relates to personal or family secrets, so use it with discretion in conversations to avoid offense. 
I didn't want to ask her about the skeletons in her cupboard during our first meeting.
Pro Tip 3/3Metaphorical Use
Understand that the phrase is metaphorical and doesn't refer to an actual skeleton or a literal cupboard. 
His charming persona hid the fact that there were quite a few skeletons in his cupboard.