Ptolemy called the straits along the Malay Peninsula "Sinus Sabaricus". What does it mean?

He was specifically referring to what we now call the Strait of Malacca, on the west side of the Malaysian Peninsula.
https://en.wikipedia.org/wiki/History_of_Indian_influence_on_Southeast_Asia#Indianised_Hindu-Buddhist_kingdoms_(3rd_century_BCE_to_14th_century_CE)

The word 'sinus' translates into English in various ways, depending on its use. The geographical sense would be bay.
https://translate.google.com/;?sl=la&tl=en&text=sinus%0A%0A&op=translate

The adjective sabaricus probably refers to it being located near Sabah (Borneo); there is a Malaysian coffee company which uses the variant form Sabarica as its name. While Sabah doesn't look that close to the strait, for someone sitting in Rome they probably seemed pretty close, and one would traverse the Strait of Malacca on a journey to Borneo.
https://en.wikipedia.org/wiki/Sabah
https://beanshipper.com/en-au/collections/sabarica