means A secret string of characters you must enter to prove you're allowed access to something, from a phone to a bank account.

from A transparent compound of "pass" and "word" — literally the word that lets you pass. It descends from military usage, where a watchword or countersign let sentries distinguish friend from foe in the dark. "Pass" comes via Old French from Latin "passus" (a step, a pace), and "word" is solid Old English "word." The term predates computers by centuries; we simply handed our ancient guard-post ritual to machines, who now demand a capital letter and a number too.