to add defensive fortifications to a site to withstand attack — as in "fortify the camp"
to make a structure or organization stronger — as in "fortify the wall"
to inspire emotional and mental strength — as in "fortify her spirits"
to strengthen an argument — as in "fortify our argument"
to add nutrients to increase the healthfulness — as in "bread fortified with folic acid and iron"
to add alcohol to make a drink stronger and/or less prone to spoiling — as in "port, sherry, madeira, and other fortified wines"
I was not sleepy, as the long sleep yesterday had fortified me, but I could not help experiencing that chill which comes over one at the coming of the dawn, which is like, in its way, the turn of the tide.