Alongside or abreast; opposite the center of the side of the ship or aircraft.
at right angles to the length of a ship or airplane
On the beam; at a right angle to the centerline or keel of a vessel or aircraft; being at a bearing approximately 090 Degrees or 270 Degrees relative.
Alongside.
The word comes from Old English, combining a- meaning "in the direction of" with beam referring to the keel. It originally described a position or movement aligned with the ship's centerline.