Luffing Definition

"Luffing" is a specialist word in English that came originally from sailing and is still perhaps most commonly used in this context.

The "luff" is the part of a sail, closest to the mast.

When a yacht turns through the wind from one tack to it's opposite, there will be a period when the sail is 'head to wind' and flaps.  This being a critical time for the manouever, sailors watch the "luff" of the sail particularly closely as it gives the first and best indication as to how things are progressing.  "Luffing" then came to mean the sail going slack and re-filling from the other side.

Hang gliders co-opted the term to describe the same process, (but in the horizontal rather than vertical) plane - which is catastrophic and often fatal for them.

"Luffing' then came to be used by sky divers, paragliders and kite fliers also.

More technically it's when the angle of attack (of a soft wing especially) becomes negative, and particularly when this happens suddenly.