Almost exclusively used in the set phrase 小首をかしげる (to tilt one's head slightly, often in puzzlement or curiosity). Rarely used independently.
彼は小首をかしげて、私の質問を考えた。
He tilted his head slightly, thinking about my question.
Compound of 小 (small) and 首 (neck/head). The exact historical derivation is uncertain, but it likely arose as a poetic or euphemistic term for the head in certain contexts.