Bailey (1731) has the meaning as : "full of weeds or reets called alga"
with the etymology shown in E-NED as: from Latin algosus from alga sea-weed.
(E-NED quotes Bailey's definition only.)
E-NED states the cold, chilly definition in E-ASH (1775) is an evident error. Ash give the etymology as 'algor'.
Johnson (1768) also shows 'algor' as the etymology.
Unfortunately, no examples were shown for either definition.