Toilet
Restroom, Bathroom, Lavatory
noun
You know that 手洗い is "handwashing" (and also a not as polite way to mention a bathroom). When you add お you're suddenly making it polite. Why? Because you're talking about something a little more crass than handwashing. You're talking about the toilet or restroom or bathroom. Ewww. Poop goes there.
おてあらい
The reading comes from 手洗い, so if you know that you'll know this too!
お手洗いは、真っすぐ行って左です。
The restroom is straight and to the left.