taken from Mastering Lightning:

"Any wallet that outsources management of keys is called a custodial wallet because a third party acting as custodian has control of the user’s funds, not the user. A noncustodial or self-custody wallet, by comparison, is one where the keystore is part of the wallet, and keys are controlled directly by the user. The term noncustodial wallet just implies that the keystore is local and under the user’s control."

It seems that non-custodial makes sense if we assume that custodial means that someone else has the custody and, therefore, non-custodial means the opposite.

Mental gymnastics achieved 🥷