inexpugnable = you can't fight your way out of it, or your eraser won't work

im[pugn 7im py1n$8
vt.
5ME impugnen < OFr impugner < L impugnare < in3, on, against + pugnare, to fight: see PUGNACIOUS6
1 orig., to attack physically
2 to attack by argument or criticism; oppose or challenge as false or questionable
—SYN DENY
im[pugn4a[ble
adj.
impugnation
n.
im[pugn$er
n.

Compare:
expunge
vt.
3punged$, 3pung4ing 5L expungere (pp. expunctus), to mark (with points) for omission, erase < ex3, out + pungere, to prick: see POINT6 to erase or remove completely; blot out or strike out; delete; cancel
—SYN ERASE