Although this phrase was apparently coined sometime ago, I hadn't heard it used until relatively recently. Prior to moving to Colorado, it was common for me to hang around lots of my younger brother's friends, which is where I first heard this remark of masculine genius. They would say it when giving each other hugs good-bye, when playing grab-ass with each other, and they would even (aware of my sexuality) sometimes tell me 'no homo,' to which I obviously would reply somewhere along the lines of, 'no, yea. I'm pretty gay.'
For those of you unfamiliar with the term and its function, wikipedia does an exemplary job of providing a formal definition which reads: "The phrase no homo is a slang-term. It parenthetically asserts the (male) speaker of such is not homosexual and is usually used after an utterance that may have given that impression (1)." In short, if a man does or says something that could be mistaken as homosexual, he simply must blurt out the phrase to retain his masculinity. Below I've included the link to a semi-crude, yet informational video in order to help you fully understand the magical 'no homo,' and its use in all contexts.
So the question now arises, why do we feel need to constantly remind our same-sex companions of our sexual intent (or lack thereof)?
In his article, 'Holy Fratrimony: Male Bonding and the New Homosociality,' Don Romesburg explains that information presented during twentieth century has conditioned society to equate love and sex, and as a result, strongly emotional or passionate relationships between members of the same sex are perceived to encompass some sexual dimension. This leaves male/male relationships to develop in two ways, either by placing sexuality at the core or by disclaiming sexual feelings (2) and therefore evading the possibility of being labeled with the stigma of homosexuality. I would say it is apparent that most male/male relationships depicted in movies, television programs, and popular culture, center on the disavowal of homosexuality. This most likely because of the fact that our society so greatly emphasizes that men participate in masculinities, with which comes homophobia. And I'm not talking about the 'I'm terrified of gay people' definition of homophobia; I'm talking about Michael Kimmel's definition of homophobia in his article, 'Masculinity as Homophobia,' whereas homophobia is defined not as a fear of gay men, but rather, a fear of being emasculated, (3) - and being perceived as a homosexuality in this society does exactly that. Thus, 'no homo' is just another tool in order to help reinforce masculinity in a hetero-normative society.
Although I would like to conclude with one more possible answer to the question of why male same-sex companions feel the need to constantly remind each other of their heterosexuality. There is the possibility that in some cases a 'no homo' could perhaps just be a method by which we repress sexual feelings towards one another. I mean consider items listed in Jennifer Baumgardner's article 'What is Bisexuality,' such as the theory of innate bisexuality proposed by Freud, or more compelling, Kinsey's study which found 50 percent of men to have had some bisexual encounter (4). Think about it, maybe a reasonable amount of people have homosexual feelings for their good friends, although the consequences of admitting such could be utterly intolerable. In any case, in order to test this hypothesis, on my next encounter with someone who drops a 'no homo' at me, I think I'm going to kiss them on the mouth and see where it goes from there - I mean, if it goes sour I can always say 'no homo' right?
(2) 'Holy Fratrimony: Male Bonding and the New Homosociality' by Don Romesburg
(3) 'Masculinity as Homophobia' by Michael S Kimmel
(4) 'What is Bisexuality' by Jennifer Baumsgardener
Photo Cred: http://knowyourmeme.com/photos/171631-no-homo#.Tpu8jXO1m_E