agger
A double tide, particularly a high tide in which the water rises to a given level, recedes, and then rises again (or only the second of these high waters), but sometimes equally a low tide in which the water recedes to a given level, rises, and then recedes again
The phænomenon of long continued high water or double high water in estuaries is, we believe, not uncommon. [...] The first high water appears to be considered by the Dutch, in common language, as the real high water, and the second is called the agger, but so little difference is there between them, that, in making the extensive series of simultaneous tide-observations proposed by Mr. Whewell, the agger was sometimes observed for high water.
Where it comes from
From Middle English agger (“heap, pile”), from Latin agger (“earthwork, rubble, rampart”), from ad- (“toward, towards”) + gerere (“to carry”).
Adapted from the Wiktionary entry agger (CC BY-SA 4.0).