Discussion and sources
Though in its origin this character, even being related to a person, has a not so clear relation with the character for person 人, now, the evolution of language made it similar. In the upper part of the person a "cubist" representation of an open mouth has been added.
This is a person yawning, or perhaps a person with the mouth open who feels a lack of air. Later it was used for other situations in which there is a lack of something, as to lack something, to be defient in something, to owe money, etc.