怦 is an onomatopoeia that imitates the sound of a heartbeat — a sudden, strong thump or pit-a-pat. You'll see it in the reduplicated form 怦怦 (pēngpēng) to describe a racing heart from excitement, nervousness, or fear. It's not used as a standalone verb; it's a sound word that pairs with 心跳 (heartbeat) or appears in the chengyu 怦然心动 (pēng rán xīn dòng, 'heart suddenly races').