Never head this term until a couple of years ago. Turns out there is not agreement as to the term “King Tide”
However NOAA says it’s another name for “Spring Tides”.
A spring tide —popularly known as a “King Tide”—refers to the ‘springing forth’ of the tide during new and full moon.
A neap tide —seven days after a spring tide—refers to a period of moderate tides when the sun and moon are at right angles to each other.
Bowditch doesn’t use the term:
Thus, when the Moon is at the point in its orbit nearest the Earth (at perigee), the lunar semidiurnal range is increased and perigean tides occur. When the Moon is farthest from the Earth (at apogee), the smaller apogean tides occur. When the Moon and Sun are in line and pulling together, as at new and full Moon, spring tides occur (the term spring has nothing to do with the season of year); when the Moon and Sun oppose each other, as at the quadratures, the smaller neap tides occur. When certain of these phenomena coincide, perigean spring tides and apogean neap tides occur.
It does point out that spring tides are not related to the seasons of the year.
Old English spring (noun), springan (verb), of Germanic origin; related to Dutch and German springen . Early use in the senses ‘head of a well’ and ‘rush out in a stream’ gave rise to the figurative use ‘originate’.