Once a…, always a…
Definition 1 of 1
Pro Tip 1/3Emotional Weight
This phrase often carries emotional or moral judgment, implying a fixed nature or inability to change. 
He may say he's changed, but once a liar, always a liar.
Pro Tip 2/3Not Literal
It's an expression, not a fact. People can change, but this phrase suggests skepticism towards that change. 
She returned the stolen money, but people say once a thief, always a thief.
Pro Tip 3/3Informal Context
Mostly used in casual conversation. It might be inappropriate in formal or sensitive situations. 
At the reunion, they joked that once a class clown, always a class clown.