The story
From oct(opus) + -opamine (“dopaminergic agent, dopamine derivative”), because it was isolated from an octopus's salivary gland.
Etymology from Wiktionary, CC BY-SA 4.0.
octopamine
Today it means: An aromatic amine, 4-(2-amino-1-hydroxy-ethyl)phenol, related to noradrenaline, having similar characteristics to dopamine. Full definition →