What does abatis or abattis mean?
The definition of abatis and abattis
Ab”a*tis, Aba”t*tis, n.
Etym: [F. abatis, abattis, mass of things beaten or cut down, fr. abattre. See Abate.] (Fort.)

A means of defense formed by felled trees, the ends of whose branches are sharpened and directed outwards, or against the enemy.