Just one more case of verbing a noun.
flank
n.
5ME flanke < OFr flanc < Frank *hlanka, akin to OHG hlanka, a hip, flank: for IE base see LANK6
1 the fleshy side of a person or animal between the ribs and the hip
2 a cut of beef from this part: see BEEF, illus.
3 loosely, the outer side of the upper part of the human thigh
4 the side of anything
5 Mil. the right or left side of a formation or force, or of the projection of a bastion
adj.
of or having to do with the flank
vt.
1 to be at the side of
2 to place at the side, or on either side, of
3 Mil. a) to protect the side of (a friendly unit) b) to attack the side of (an enemy unit) c) to pass around the side of (an enemy unit)
vi.
to be located at the side: with on or upon