'anti-syphilitic' is formed in English from the prefix 'anti-' (from Greek 'anti' meaning 'against') combined with 'syphilitic', itself derived from 'syphilis' plus the adjectival suffix '-ic'.

'syphilis' entered English from New Latin 'syphilis', named after the fictional shepherd 'Syphilus' in a 16th-century poem by Girolamo Fracastoro; the adjective-forming element comes via Latin/Greek suffixes ('-icus' / '-itikos') that produced 'syphilitic', and the combining prefix 'anti-' was appended to indicate opposition to the disease.

The compound originally (and consistently) meant 'against syphilis' and has retained that medical sense as referring to treatments or agents active versus syphilis.