'plethoric' originates from New Latin 'plethoricus', ultimately from Greek 'plethorikos' where the root 'pleth-' from Greek 'plethos' meant 'fullness' or 'multitude'.

'plethoric' passed into English via New Latin/medieval Latin forms such as 'plethoricus' (and was influenced by French 'pléthorique'), eventually becoming the modern English adjective 'plethoric'.

Initially it meant 'abounding in fullness' (especially fullness of blood); over time the sense broadened to mean 'excessive' or 'overabundant' in general.