HEADS – the lavatory. Always pronounced in the plural.
In the days of sail, the wind would ideally come from astern or the quarters (45 degrees from astern). Therefore it was prudent to go to the head of the ship (bow) to do your business (remembering the saying "don’t piss into the wind").
You had a choice of which side of the bow to use and this is why it’s referred to in the plural. (only the USN uses the singular ‘head’)