What word did the Gileadites make men say at the Jordan to expose Ephraimites?

Answer: Shibboleth

Ephraimites said "Sibboleth" because they could not pronounce it correctly, and they were seized at the fords of the Jordan.

“they told him, “Please say Shibboleth.” If he said, “Sibboleth,” because he could not pronounce it correctly, they seized him and killed him at the fords of the Jordan. So at that time 42,000 Ephraimites were killed.”— Judges 12:6 (BSB)