means To begin doing or experiencing something, or the point at which that beginning happens.

from From Old English 'styrtan,' meaning to leap up or move suddenly — the violent jolt of a body springing into motion. That sense survives when we 'start' awake or 'start' in fright; the calmer meaning of 'commence' grew out of that same sudden movement. It's likely related to a Germanic root for 'tail' or 'jutting thing,' the idea being something thrust abruptly outward.