Mainly U.S.: medieval vernacular form of Randolf. This was in common use as a given name into the 17th century and gave rise to a surname. In modern use the given name is often a transferred use of this surname.
Dutch: from keyser ‘emperor’, modern Dutch keizer (from Latin Caesar). Keyser is a 17th-century spelling. Compare German Kaiser. In addition to the meanings of the German name (nickname, house name, etc.), this was also a status name for the head of a society of marksmen.