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.
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.
ジェームスはいつも、お手洗いに行く前にウィスキーを一口飲む。
James always takes a sip of whiskey before going to the bathroom.
お手洗いで用を足しているうちに、座ったまま寝てしまいました。
I dozed off while sitting in the bathroom doing my business.