The “Line of Death” was a boundary that Libya’s Qaddafi delimited in the Gulf of Sidra. Anyone trespassing without authorization would be eliminated — thus the Line of Death. Reagan only laughed, and attacked Qaddafi where he lived. Barely escaping death, not much was heard of Qadaffi for the nest 18 months.
Hence the classic Robin Williams' routine:
"This is a 'Line of Death.' - You cross it, you die!"
"Ok, here is another 'Line of Death - You cross it, you die!"
"Here's another line."
"Another line."
"Alright that's it, you come a-knocking on my door, I'm not coming out."