Dennis Baron of the Oxford University Press blog explains the misunderstood origins of the honorific “Ms.”:
The form goes back at least to the 1760s, when it served as an abbreviation for “Mistress”… and for “Miss,” already a shortened form of “Mistress,” which was also sometimes spelled “Mis.” The few early instances of “Ms.” carried no particular information about matrimonial status (it was used for single or for married women) and no political statement about gender equality… While “Miss” was often prefixed to the names of unmarried women or used for young women or girls, it could also refer to married women…
Ms. didn’t really take off until the politically-motivated language reforms of second-wave feminism and the cultural impact of Ms. Magazine in the 1970s. Many of the form’s popularizers at that time thought of Ms. as a blend of Miss and Mrs., but some evidence suggests that it derives more directly from Miss, or possibly from Mistress.
The earliest form is from a 1767 tombstone of Ms. Sarah Spooner. The “Ms.” was just a shortened version of “Mistress” by a stonecutter trying to save space on the already crowded slab.