Acabar — the everyday Spanish word meaning “just”, “finish”, and, wait for it “to ejaculate” (don’t ask how I learned the last definition!) — comes from the Latin caput, meaning “head”.
Thus, it has a parallel in the English expression: to bring to a head. Although that phrase doesn’t exactly mean to finish (it means, to force a decision to be made, basically), it is a similar concept: bringing about a totality that finishes or just about finishes something that had been happening.
Thus etymology proves the common sense wisdom that, it’s easy to start something… but it requires real intelligence, a head, to finish what you start.