P-trap

A P-trap is the U-shaped bend of pipe under a sink, basin or floor drain that always holds a small plug of water. That water seal is what stops foul sewer gas drifting back up into the room — so a drain that suddenly smells bad often just has a dry or empty trap, not a blockage. In the UAE this bites hardest in guest bathrooms and rarely-used floor drains, where the heat evaporates the seal within a week or two; running the tap for a few seconds usually refills it. If the smell survives that, the venting or the trap itself needs a look — a quick job for a licensed plumber.