source: [ゆゆ式の日向縁が櫟井唯の脇腹を突くGIF画像｜無料GIF画像検索 GIFMAGAZINE 100623]

1. the side of a person's or animal's body between the ribs and the hip:

(動物･人の)脇腹, 横腹〘ろっ骨と尻との間〙

e.g. leaning against his horse's flanks.

a cut of meat from the flank of an animal:

(牛などの)脇腹肉の切り身

e.g. a thick flank of beef on a spit

e.g. two pounds of flank.

the side of something large, such as a mountain, building, or ship:

｟かたく｠ (山･建物の)横側, 側面(side)

e.g. the northern flank of the volcano.

2. the right or left side of a body of people such as an army, a naval force, or a soccer team:

(戦場の部隊の)側面; (サッカーなどの)サイド(wing)

e.g. the left flank of the Russian Third Army.

the right or left side of a game board such as a chessboard.

(チェス盤などの左右の)側

be situated on each side of or on one side of (someone or something):

〈人･物〉の(両)側に立つ, ある, 位置する (!しばしば受け身で)

e.g. the fireplace is flanked by built-in bookshelves.

(usually as adjective flanking) guard or strengthen (a military force or position) from the side:

〘軍〙 …を側面を守備する; …の側面を迂回（うかい）する

e.g. massive walls, defended by four flanking towers.

(usually as adjective flanking) attack down or from the sides, or rake with gunfire from the sides:

〘軍〙 …を側面攻撃する

e.g. a flanking attack from the northeast.

late Old English, from Old French flanc, of Germanic origin.