draw a line in the sand

To establish a figurative boundary that someone or some group refuses to cross or beyond which no further advance or compromise is accepted. I don't mind my roommate being a bit messy, but leaving dirty dishes for me to clean up is where I draw a line in the sand! I'm willing to accept some minor edits on my script, but I draw a line in the sand at any major rewrites. Aunt Peggy was fine with us setting her up on a date, but she drew a line in the sand at online dating.

Fig. to create or declare an artificial boundary and imply that crossing it will cause trouble. Todd drew a line in the sand by giving his roommate an ultimatum about his sloppiness—he had to start cleaning up after himself or move out.