The only place I find it is in third party references, although I found this in the Catholic Encyclopedia:
Within the Church’s teaching on human sexuality, masturbation is considered to be objectively disordered or intrinsically evil (in se malum or intrinsece malum); that is, no circumstances or intentions can render the action of masturbation “morally good.” The degree or seriousness of the moral culpability of a particular act of masturbation, however, can be judged only in light of the degree of the moral agent’s knowledge, freedom, and intentions. Full moral guilt requires a fully deliberate choice of what the person sufficiently realizes to be seriously evil, which can then be evaluated as gravely, or mortally, sinful. If the act is performed with only partial realization or only partial choice of the will, the person is presumed to be guilty only of venial sin. “One must take into account the affective immaturity, force of acquired habit, conditions of anxiety or other psychological or social factors that lessen or even extenuate moral culpability” (CCC, n. 2352).
But I can’t find that phrase in anything published by the Church herself, no matter how many ways I Google the thing.
Wiki says the Latin phrase can indeed mean “bad” or “evil” - although Wiki seems to show that the syntax as provided above is incorrect.
en.wikipedia.org
However, I seriously cannot equate the act with other things we
know are evil - like murder, for example. I can’t logically believe that the penalties for these two things are the same.