noun
overhear; happen to hear
Used almost exclusively in the set phrase 小耳に挟む (to overhear, to catch a bit of information). The noun itself is not freely used.
喫茶店で隣の人の会話を小耳に挟んだ。
I overheard the conversation of the person next to me at the café.
彼が転職するらしいと小耳に挟んだ
I happened to hear that he's changing jobs—is that true?
立ち聞き means intentionally eavesdropping, while 小耳に挟む implies unintentionally overhearing something.
Compound of 小 (small, little) and 耳 (ear). The phrase 小耳に挟む literally means 'to catch in one's little ear,' suggesting a casual, incidental hearing.