past participle of lie (“to be oriented in a horizontal position, situated”)
"He has lain his true feelings about the scandal to rest until he is ready to speak again."
To conceal, keep quiet about.
A surname.
"The new headmaster at Oakwood Elementary is a Mr. Lain who has been teaching history for thirty years."
The word "lain" is simply a past participle form of the verb "lie," meaning to rest or recline. It does not have a separate origin story because it evolved directly from the same root as its base verb.