(chess) a chess pawn that is moved two squares can be captured by an opponent's pawn commanding the square that was passed
A move in which a pawn captures an opposing pawn on the same rank immediately after the latter has moved forward two squares on its first move in the game, as if it had moved forward only one square.
incidentally; in the course of doing something else
"he made this remark in passing"
In passing, by the way, incidentally.