fas·tig·i·um 
(fă-stĭjē-əm)
(fă-stĭjē-əm)n.
The period of maximum severity or intensity of a disease or fever.
[Latin fastīgium, apex, height.]
- ontogenetically
- ontogenic
- ontogenically
- ontogeny
- ontological
- ontologically
- ontologist
- ontology
- on top
- on top of
- on top of the world
- on trial
- on/under pain of
- on/upon(one's) hands
- on/upon (one's) hands
- on/upon request
- on/upon the heels of
- on uppers
- onus
- on view
- on vinyl
- on waivers
- onward
- on wax
- on way