Wiktionary
n. (context legal English) A trust that is created for the benefit of a person (often because that beneficiary is unable to control spending) that gives an independent trustee full authority to make decisions as to how the trust funds may be spent for the benefit of the beneficiary, thus keeping the funds in the trust beyond the reach of creditors.
WordNet
n. a trust created to maintain a beneficiary but to be secure against the beneficiary's improvidence
Wikipedia
A spendthrift trust is a trust that is created for the benefit of a person (often unable to control his spending) that gives an independent trustee full authority to make decisions as to how the trust funds may be spent for the benefit of the beneficiary. Creditors of the beneficiary generally cannot reach the funds in the trust, and the funds are not actually under the control of the beneficiary.