The word "toilet" is, itself a euphemism. Initially (i.e., when imported into the English language from the French circa 1660), it referred to matters of dress and cleanliness. To "see to ones' toilette" referred to tidying up and dressing to meet the day. Or it meant that one must go make repairs in one's "toilette," as in when milord's powdered wig gets knocked askew or the whalebone in milady's corset is uncomfortably poking one of her breasts up out of her décolleté. It became a handy euphemism for excusing oneself from the gathering in the drawing room to do a quick hunt for the nearest chamber pot*.

Vestiges of the original meaning continue today when referring to such things as soap, razors, make-up, perfume, et al. as "toiletries."

Don Firth

*Later known as a "thunder mug." (Not a euphemism.)