I assume you know already how spring tides and
leap tides occur and you only want to know where
they got their name from.
Well, this is a tough question. As you know,
the sun and moon each raise tidal bulges on the
Earth's oceans. When the sun and moon are on the
same side of the Earth (new moon) or the opposite
side (full moon), their bulges add together to
make larger tides than usual: this is called a
"spring tide" . When the sun and moon are
90 degrees apart (first quarter or last quarter
moon), the bulges interfere and cancel each other:
this creates the unusually small "neap
Now, especially high tides are called
spring tides, but they have nothing to do
with the season and actually occur twice a month.
I think the name comes from the German word
"Springen" meaning "to leap". Spring tides
also mean lower low water. The opposite to a
spring tide is a neap tide.
The derivation of neap tides,
however, seems simpler. The Shorter Oxford
dictionary gives the derivation of 'neap' from the
Old English (OE) word 'nep', to become lower.
There is an article on the WWW that tries
to explain the roots of the words "spring" and
"neap". The author basically comes to the same
conclusion although she believes "spring" comes
from the Anglo-Saxon word springan (to bulge). But
in the Middle Ages the German language and the
Anglo Saxon were very close.