adj. Mildly pejorative contraction of postmodern.

Example Citations:
But overall, these sisters are sick to death of the bad rap their brand of feminist politics is getting from their arch-enemies, the academic postmodernists—also known, none too affectionately, as “po-mos“.
—Elayne Rapping, “Politics and polemics,“ The Woman‘s Review of Books, October 1, 1996

As a result, Ms. Turkle suggests, the Internet has become the embodiment of “postmodernism“ or “po-mo“ as many college students now call it.
—Edward Rothstein, “If life on the Web is postmodern, then maybe Foucault really was a Power Ranger,“ The New York Times, April 1, 1996

Earliest Citation:
Essentially, what Gruft considers the new architecture has a pronounced Post-Modernist slant even though he insists he isn‘t making the “usual superficial distinction drawn between Modern and Post-Modern.“

Yet virtually all the characteristic features of “po-mo“ are found in these buildings. We see, for example, exterior walls painted pastel pink, big towers topped by little pointed roofs, ziggurat shapes and lots of glass.
—Christopher Hume, “Exhibit probes ‘new‘ Canadian architecture,“ The Toronto Star, December 20, 1986