Lifer
/ˈlaɪfər/
Definition 1 of 2
Pro Tip 1/3Life Commitment
Refers to a long-term dedication, whether to a job or due to a prison sentence. 
My uncle is a lifer at the postal service; he's been there for 35 years.
Pro Tip 2/3Negative Connotation
When related to prison, 'lifer' can have a negative or heavy feeling, signaling no release. 
The jury's decision made him a lifer, without the possibility of parole.
Pro Tip 3/3Job Loyalty
In work context, being a 'lifer' can suggest dependability and loyalty to an employer. 
She's a lifer at the bank; her colleagues respect her commitment.