“like an Arab in the desert.” This refers to the custom of some of the Bedouin Arabs who robbed for a living. For millennia the Bedouin Arabs have been known to lie in wait for travelers and then rob them. Pliny the Elder wrote about them some 2000 years ago (Natural History, vi. 28) and many books on the culture and customs of the Bible Lands from the 1800s and 1900s mention the fact.
Jeremiah compares the people of Judah and Jerusalem (and Israel, although Israel had been gone for over 100 years) to an Arab in the desert. Just as the Arab waits for unsuspecting travelers to rob, Judah sits and waits for more idol gods to satisfy her lust.