The side of the torso; the flank (the area between the chest and the waist on the body's side).
走っているときに脇腹が痛くなってきた。
My side started to hurt while I was running.
転んで脇腹を打ったので、しばらく呼吸が苦しかった。
I fell and hit my flank, so breathing was difficult for a while.
シャツの脇腹に穴が開いている。
There's a hole in the side of the shirt.
An illegitimate child; a child born to a woman other than the legal wife (archaic/colloquial usage; somewhat dated).
古い文献では『脇腹の子』という表現で私生児を指すことがある。
In old writings the phrase 'child of the flank' is sometimes used to mean an illegitimate child.