means shaking with small, involuntary movements, usually from fear, cold, weakness, or strong emotion.

from From the verb "tremble," which came into English through Old French "trembler," descended from Latin "tremulare" — itself rooted in "tremere," to shake or quake. That Latin "tremere" is the same nervous ancestor behind "tremor," "tremulous," and "tremendous" (literally "to be trembled at"). The "-ing" simply turns the shaking into something ongoing, a verb caught in the act.