'astacian' originates from New Latin, specifically the word 'astacius', where the combining form 'astac-' (from Greek 'astakos') meant 'crayfish'.

'astacian' developed via Medieval/Neo-Latin forms such as 'astacius' and entered English as a technical adjective and occasional noun; the element ultimately comes from Greek 'astakos'.

Initially it referred directly to the animal 'crayfish'; over time it has been used adjectivally ('relating to crayfish') and, in some literature, as the name of a regional geologic stage ('Astacian').