I suggest this one-word expression to help you avoid having an “accident” before someone figures out what you are trying to say. For a more specific suggestion or a fuller expression, we will need more specific context. For example, are you asking for directions, requesting permission, or simply excusing yourself? More importantly, what is your target dialect: Egyptian, Lebanese, Bahraini, Yemeni?
When I was growing up in Qatif, Saudi Arabia, we used the term AL-ADAB or BAIT AL-ADAB (“house of manners”). The term refers to the ritualized manners associated with the use (and sharing) of toilet rooms. Those who know Arabic can imagine the jokes we had about AL-ADAB AL-ARABIYY and AL-ADAB AL-INGILEEZIYY.
In other parts of Saudi Arabia and the Gulf region, the term AL-KHALA or BAIT AL-KHALA (“house of privacy”) was used. The term is self-explanatory.
Such indigenous terms have been largely replaced by terms imported from Turkey, France, and the UK. Examples:
1. HAMMAM is a Turkish term meaning “bath house.” It has been so fully integrated into Arabic as to engender a number of verbs: HAMMAMA, TAHAMMAMA, ISTAHAMMA
حَمَّم، تَحَمَّم، استَحَمَّ
The late Syrian poet Nizar Qabbani used these verbs in many of his poems. The extension of the term to the “rest room” was almost inevitable, parallel to the use of “bathroom” in English.
2. TWALAIT is clearly derived from the English “toilet,” which comes from the French “toilette,” although French usage carries very different meanings.
3. DOWRAT AL-MIYAH (DORT EL-MAYYA in EGYPT) is a loose translation of the British “W.C.” This expression became popular when running water and sanitary sewer systems were introduced into remote regions, in some cases with a great deal of political fanfare. In my town, we didn’t have running water until the early fifties of the last century, and didn’t have sewer systems until the late sixties or early seventies.
Many languages refer to the toilet using modified imported terms, mostly to avoid making a direct reference, which may sound too vulgar. For instance, the British expression “the loo” comes from the French expression “Gardez l’eau” (“watch out for the water”), which was shouted out before the chamber pot content (more than just “water”) was tossed out of the window into the street at night. Just before dawn, hungry pigs were let loose to consume the human refuse. The expression came into England with the Norman Conquest of 1066, was modified to “gardy loo,” and then became “the loo.”
Arabic is no exception.
AL-HAMMAM will serve your most immediate purposes in most Middle Eastern countries.
MIRHADH is a modern coinage, and a very successful one. It refers to the “John” itself (as in construction specifications), but is also used for the “rest room.”