An interesting military term that I don’t recall seeing before. Also of note is that only the most recent reference given by the OED (out of six) uses the spelling given by the OED. The rest either double the “b” or the “t”.
abatis
Milit.
A defence constructed by placing felled trees lengthwise one over the other with their branches towards the enemy’s line, and piling them up until a shelter for workmen is obtained.
Neh 