under one's belt
in one's experience/memory/possession, gained by effort and skill

[TahlilGaran] Dictionary of Contemporary English ▲

under belt
under (your) belt
already achieved or experienced.
She had fourteen years as a manager under her belt and knew the job as well as anyone.

He has several literary awards under his belt.

[TahlilGaran] English Idioms Dictionary ▲

under one's belt
1. in one's experience/memory/possession, gained by effort and skill
Now that I have some job experience under my belt I will have more chances to apply for a better job.

When he has more experience as a cook under his belt he will begin to look for a job.

2. in one's stomach
After he had a big breakfast under his belt he was ready for work.

[TahlilGaran] English Idioms Dictionary ▲

under one's belt
adv. phr., informal
1. In your stomach; eaten; or absorbed.
Once he had a good meal under his belt, the man loosened his tie and fell asleep.
Jones is talkative when he has a few drinks under his belt.
2. In your experience, memory or possession; learned or gotten successfully; gained by effort and skill.
Jim has to get a lot of algebra under his belt before the examination.
With three straight victories under their belts, the team went on to win the championship.

[TahlilGaran] English Idioms Dictionary ▲