What is the meaning of "khasiratahu" (his flank/side)?

Ibn Manzur defined the khasira (flank/loin) in "Lisan al-Arab" as: "what is between the hurqafah (hip bone) and the qusaira (lower ribs), and it is found in humans and animals." Malik narrated in Al-Muwatta' that Yahya bin Sa'id said: "I was walking, and I was struck by a khasira (pain in the flank), so I rode," meaning his flank pained him. The poet says: "When we gave it the ʿakees (a type of drink), its flanks swelled and its jugular vein increased in pulsation," referring to the fullness of their bellies.