The duty of watching patrolling, etc., performed by a sentinel or sentinels.(noun)
Gnu Collaboartive International Dictionary of English: licensed under The Code Project Open License (CPOL)
Use "guard duty" in a sentence
"His Excellency is very short of free men to act as supervisors of convicts because Major Ross has let it be known very loudly that the Marine Corps is not about to extend guard duty to supervisory duties."
" The next time Lurlinemas rolled around, Liir volunteered for solitary guard duty in the watchtower that capped the great chapel."
"Finally, in one quick stroke with no time to debate the issue, the king had severed Parmenion from his soldiers and cashiered the old man to guard duty away from the army."