under one's belt
Opinions vary  between:
- Comparing the experience and what was learned and gained from it, to a meal now eaten and safely stowed away in the stomach.
- The effect of a punch below the belt in boxing or other fist fighting.
- Already done; within one's experience; practiced.
- 1998, Ani DiFranco (lyrics), “Two Little Girls”, in Little Plastic Castle, performed by Ani DiFranco:
- You were fresh off the boat from Virginia / I had a year in New York City under my belt / We met in a dream, we were both nineteen
- He got off to a shaky start, but with a few months of experience under his belt, he kept up handily with the veteran employees.
within one's experience