This thermos is advertised as completely leakproof, even when shaken vigorously.

leakproof 是由名词 leak（泄漏）和形容词 proof（防...的）组成的复合形容词。proof 作为后缀时表示"防...的、抗...的"，如 waterproof（防水的）、fireproof（防火的）。leakproof 合起来表示"防漏的、不漏的"。