Queer originally just meant "weird," but it's unfortunately evolved into an offensive term for "homosexual."
Like many words, queer has many meanings. One is "odd," as in "I feel kind of queer today." The other is "gay." Calling someone queer is a slur: it has about the same meaning and level of offensiveness as fag. As a verb, queering can mean to endanger or expose to risk, and also to hinder, thwart, foil, or baffle. As usual, you can figure out which meaning of queer someone intends by the other words around it.