IMHO is an acronym for "In My Humble Opinion." It came into being in the early days of the Internet and chat rooms. Participants in online conversations, often unused to typing all their thoughts instead of simply expressing them orally, developed a wide range of textual shortcuts for common expressions. IMO, meaning "In My Opinion," and IMHO were among the earliest, along with brb, or "be right back," which was used to alert participants that one would be absent from the conversation for some period of time.
“In my humble opinion,” of course, isn’t actually a commonly used expression outside Internet conversations. It developed as a way for users to moderate the things they said online, the same way facial expressions, hand gestures and other non-verbal cues help to moderate the things people way when engaged in actual conversations. When the Internet's pioneers began using online chat rooms, it quickly became apparent that there were significant differences between online and "analog" forms of communication, such as face-to-face or telephonic. They quickly developed the shorthand that’s become ubiquitous in the subsequent decades to try to make up for the loss of non-verbal cues, as well as simply for the sake of convenience. This reinforced the understanding that in human communication, less than half the actual information transmitted is conveyed in the words used.
Online communication didn’t evolve as a keyboarded translation of verbal communication. Even discounting the importance of non-verbal cues, “normal” conversational human communication is rambling and often consists of a great many filler words and expressions, such as “ah” and “um.” In addition, it takes much longer to type most words out completely than it does to say them. Online conversations quickly developed into a combination of acronyms and shorthand, both to shorten the time necessary to express thoughts and to ameliorate the fact that nonverbal cues were non-existent. This evolution continued more rapidly with the advent of cellular telephones and their capability for text messaging.
Among the first Internet acronyms was LOL, or “Laughing Out Loud”. LOL came about because there was no standard for expressing non-verbal sounds, such as laughter, in online communication. Once LOL entered the online lexicon and gained some measure of acceptance, users began to build on it, quickly establishing ROFL, or "Rolling On the Floor Laughing." ROFLMAO and other types of expressions with obscenities quickly followed.
One Internet acronym that really did take the place of a commonly used expression was brb, which for some reason was usually typed lower-case. It signaled to other participants in a discussion that a user would be absent for a while, and was necessary because, since they couldn’t see each other, nobody could tell when one of the participants got up and left the conversation or became actively engaged in other pursuits. A similar code was afk, which means "away from keyboard." Both of these terms, and some others, evolved as measures of courtesy so that others wouldn’t sit staring at their screens, awaiting responses from people who weren’t there.
IMO and IMHO, for their part, developed as ways for users to “tone down” what might appear to be strident or overenthusiastic assertions. Emoticons, those combinations of punctuation symbols meant to indicate facial expressions like smiles, frowns or winks, gained a degree of legitimacy among Internet conversationalists for the same reason. The terse nature of Internet communications often makes expressions of opinion appear to be declarations of absolute truth, incontrovertible on their face. For example, “NYY r bst bball team evr” — “New York Yankees are the best baseball team ever” — might seem hyperbolic to some. IMO, IMHO or an emoticon like ;) are often added to such assertions to make them less strident, less forceful, and to remind others that the declaration is, after all, just an expression of opinion.
After the first bold steps in Internet communication and the development of those first acronyms, some have become commonplace in written and even verbal communication. Instead of laughing out loud, for example, someone might now respond “Lol” to a humorous remark. Likewise, while debates about the capabilities and strengths of different sports teams will rage in taverns and living rooms until the end of time, from time to time one will hear such debates punctuated with, “IMO, of course ... IMHO.”