It's impossible to know what they mean without further info., a picture of the animal, etc., but I'm tempted to think that it is a shear wall (one resisting shear or cisaillement not only by being suitably reinforced but also by being perpendicular to the wall that has been partially demolished.

A wall will be weakened if an opening is made in it, so it would stand to reason that two shear walls would not only tidy up the opening and, together with the heavy-duty lintel, make it nicely square or rectangular, but would also improve its structural integrity.

Of course they need not be perpendicular to the main wall to achieve this, but their effect would be greater if they were.

Fermeture in this context would refer to making the ends of the existing wall, where the opening has been made, neat and tidy. This could be done with the thickness of each new perpendicular shear wall. The shear walls need not be of any great length, and be something akin to a buttress, not necessarily any longer than the lintel is broad.

But as I say, this is guesswork.