Used as a noun, a "flank" is the end (or side) of a military position, also called a "wing".  An unprotected flank is "in the air", while a protected flank is a "refused flank".  Used as a verb, "to flank" is to move around and gain the side of an enemy position, avoiding a frontal assault.

late Old English flanc "flank, fleshy part of the side," from Old French flanc "hip, side," from Frankish or another Germanic source, from Proto-Germanic *hlanca- (source also of O [..]

1590s (military), "to guard the flank," also, "to menace the flank, fire sideways upon," from flank (n.). Meaning "stand or be placed at the side of" is from 1650s. Relat [..]

side of something.

to be or place at the side of something.

the side of military or naval formation; "they attacked the enemy's right flank" be located at the sides of something or somebody a subfigure consisting of a side [..]

The right or left end of a military formation

Area between the horse's ribs and hip. Lacking heavy musculature and the site of important internal organs, the flank is a very sensitive region on the horse's body and cannot be touched by [..]

The abdominal wall of an animal or carcass; is a wholesale cut of a beef carcass.

A cut of beef taken from the abdominal muscles.

(n) the side of military or naval formation(n) a subfigure consisting of a side of something(n) a cut from the fleshy part of an animal's side between the ribs and the leg(n) the side between rib [..]

latus -eris

The working surface of a gear tooth, located between the pitch diameter and the bottom of the teeth

Area between the horse's ribs and hip. The flank lacks heavy musculature, and is the site of important internal organ. Therefore, the flank is a very sensitive region on the horse's body and [..]

The side of the body between the last rib and the hip.

To come round the side

Area between the belly and the wings, more posterior.

(USN) The maximum speed of a ship. Faster than "full speed".

The maximum speed of a ship. Faster than "full speed".

The maximum speed of a ship. Faster than "full speed".

The maximum speed of a ship. Faster than "full speed".

The side of the body between the last rib and the hip.

the side of the trunk of body on the sword-arm side

To attack an enemy from their side or rear by moving through an alternative route instead of engaging them from the front.

the 10 - 15 yards from the side lines, often where crosses come in from

When an enemy attacks from the 'Flanks', they're coming from, or moving along, the outside edges of the battlefield. The 'Flanks' on a Chess Board are the Queenside Files: a, b and c; and the Kingside Files f, g, and h, respectively.

the left and right sides of an army, or the verb for attacking from the sides; commonly misused for attacking from behind.

To attack from the side or rear, where an enemy's armor is weaker. Can also refer to a contested region on the mini-map.

The files that do not belong to the centre, that is the a, b and c files on the queenside, and the f, g and h files on the kingside. Certain openings that focus on flank development are called "f [..]

(n.) The left and/or right side(s) of a target, for attacks with positional bonuses.

To flank is to engage a group of enemies from a different angle from another group of teammates, at the same time as that other group of teammates. Successful flanks can be very difficult to pull off.