Grammaticalization refers to the reanalysis of content words in a language to act as function words over generations of speakers.
Some languages have "grammatical" personal pronouns, with six to nine terms based on person, gender, number, and clusivity (you and I vs. she and I) in first person. Others have an open class of nouns used as first and second person pronouns, which become grammaticalized over time, sometimes phonetically eroded, and sometimes losing their noun meanings. Nouns pressed into service as personal pronouns come from five sources: forms of address to rulers or other superiors (such as "highness" or "your grace"), other social status terms, kin terms, professions, and generic words for "person" or age or sex terms. Nominal pronouns need not be fixed to a first, second, or third person. Southeast Asian lowland languages' pronouns in particular are subject to a euphemism treadmill, where the previous first and second grammatical pronouns become seen as intimate or rude, and new polite ones are formed from "servant" and "lord/master".[1] Likewise in Japanese, with words meaning "private" or "servant" for "I", and words for "lord", "that part", or "front" for "you". [2]:42
Sometimes the meaning of a function word is reinforced by a content word. Examples include French ne {verb} pas for "don't {verb} [a] step" or Italian che cosa for "what thing". Eventually the function word erodes away, leaving only a content word to act as a function word: pas for "do not" or cosa for "what".[2]:53 This is called Jespersen's Cycle.