'bacillar' originates from New Latin, specifically the word 'bacillaris', where 'bacillus' meant 'little rod' or 'small staff'.

'bacillar' changed from the New Latin word 'bacillaris', derived from the Latin noun 'bacillus', and entered modern English through scientific Latin usage (parallel to 'bacillary').

Initially, it meant 'pertaining to little rods (bacilli)', but over time it evolved into its current meaning of 'relating to or caused by bacilli'.