Similar to other countries, Singapore had begun to explore how to integrate computational thinking (CT) into its school curriculum through the introduction of coding lessons, computing subjects, as well as integrating CT into existing subjects. Many definitions of CT were either based on computer programming, or broadly formulated as thinking processes for problem solving. In order to authentically integrate CT into the mathematics subject, we wondered if there were meanings of CT specific to the discipline. We interviewed six teachers who taught both mathematics and computing (grades 7-12) to uncover what they believed CT meant in the two subjects. Using deductive and open coding of the transcripts, we surfaced a key finding, which was that teachers' ideas about CT in computing were strongly influenced by computer programming while their ideas about CT in mathematics corresponded with familiar ways of teaching and learning mathematics. This sense of the "familiar" appeared to be based on associating broad definitions of CT with existing mathematical thinking (MT), which made CT and MT practically identical in the teachers' minds. Teachers who found CT familiar in mathematics were more likely to favor "unplugged" approaches. These findings inform education stakeholders who support a constructionist approach—in which children program "objects" that they reflect on and share with others—on how to work with teachers. We make the following two recommendations: (1) make the familiar less familiar by drawing teachers' attention to differences between disciplines that influence meanings of CT, and (2) make the unfamiliar more familiar by engaging teachers in programming activities that connect with core ideas in mathematics.