I've seen it spelled achatmat, achatma, chakmak....
...and many other versions, with or without glotas.
So how does one spell the word? And what is its origin?
Maybe answering the second question will shed light on the first.
The word is made up of two root words.
The first is ACHA. Acha means "equal, the same, similar, also."
Acha baba i dos. The two are equally bad.
Acha amko' hit. We are the same age.
Acha poddong i lepblo yan i båso. The book and the glass fell at the same time.
The KMA' that follows is a contraction (shortening) of the word guma'.
In other words, acha guma' becomes achakma'.
It means two people living (romantically) in the same house. But they aren't married.
So the older meaning of achakma' was a concubine, a lover you lived with. Not a lover whom you met every so often on the sly, and who lived apart from you.
We know this from the root word guma'. This kind of lover lived in the same (acha) house (guma') with you.
Today, achakma' is more understood as a lover or mistress who is "on the side," not one you live with openly. Language is always evolving and changing.
But the older meaning is a man and a woman who skip marriage altogether (whether religious or civil) and live together under the same roof, in the same house (guma').
The man and woman here skip over both the Justice of the Peace and the clergy man.
* By the way, many older Chamorros would also use the word konkubino, or concubine, for two people living openly in a romantic partnership without marriage.