Tarāʾib is the plural of tarībah and refers to the upper chest region, around the collarbones or the space in front between the shoulders. It describes a region rather than a single narrowly defined bone.
The suffix -hā in tarāʾibuhā refers to the woman. Joined with maṣqūlah, it evokes a smooth, lustrous surface. A non-rational plural may take a singular feminine predicate or adjective in standard Arabic usage.
The grammatical interpretation needs both the anatomical reference and the agreement pattern. Translating the whole expression simply as “chest” loses some specificity.