胫 is a bound form (b.f.) that appears in literary, medical, and anatomical compounds. Its core meaning is the shin or lower leg, especially the tibia bone. By itself it doesn't function as an independent word; you'll encounter it in terms like 胫骨 (tibia) and the set phrase 不胫而走 (to spread like wildfire, literally 'runs without legs'). It is not equivalent to the standalone word 小腿 'lower leg'.