Crossword Clue:

that cannot be crossed

AnswerCrossword Clue
INDELIBLEThat cannot be forgotten
TRESSLock that cannot be opened
DRAFTCall to arms (that cannot be denied).
FORDABLEThat can be crossed
HARDLIMITBoundary that absolutely can't be crossed
FIBSomething that might be said with fingers crossed behind the back