expression, godan verb (-tsu)
Used especially for ears standing up sharply, often in response to a sound or alertness. The phrase ピンと立つ is a fixed expression combining the mimetic adverb ピンと (describing a sharp, straight, or tense state) with the verb 立つ.
See also: ピンと
犬が音に反応して耳をピンと立てた。
The dog perked up its ears at the sound.
猫は警戒して耳をピンと立たせている。
The cat is on alert, with its ears pricked up.