。
She hid her disappointment behind a laugh.
Literal
She [topic-は] disappointment [of-の] feeling [object-を] laughing hid.
押し隠す (おしかくす) is a compound verb meaning 'to suppress/conceal' — 押す (push) + 隠す (hide), suggesting active effort to keep something hidden. 失望の気持ち (feelings of disappointment) is the object being concealed. 笑って (by laughing) uses the te-form to show the method of concealment. A psychologically rich sentence about emotional masking.