In university mathematics education, students do not simply learn mathematics but are shaped and shape themselves into someone new - mathematicians. In this study, we focus on the becoming of "dis"abled mathematical subjects. We explore the importance of abilities in the processes of being and becoming in university mathematics. Our interest lies in how teaching and assessment practices provide students with ways to understand themselves as both able and "dis"abled, as "dis"abilities are only understood with respect to the norm. We analyse narratives of nine university students diagnosed with learning disabilities or mental health issues to investigate how their subjectivity is constituted in discourse. Our analysis shows how the students are shaped and shape themselves as "dis"abled mathematicians in relation to "speed" in mathematical activities, "disaffection" in mathematics, "individualism" in performing mathematics, and "measurability" of performance. These findings cast light on the ableist underpinnings of the teaching and assessment practices in university mathematics education. We contend that mathematical ableism forms a watershed for belonging in mathematics learning practices, constituting rather narrow, "normal" ways of being "mathematically able". We also discuss how our participants challenge and widen the idea of an "able" mathematics student. We pave the way for more inclusive futures of mathematics education by suggesting that rather than understanding the "dis" in disability negatively, the university mathematics education communities may use dis by disrupting order. Perhaps, we ask, if university mathematics fails to enable accessible learning experiences for students who care about mathematics, these practices should indeed be disrupted.