under one's belt

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.