The story
In reference to the Smurf cartoon characters, suggesting a large number of small attackers overwhelming a larger opponent.
Etymology from Wiktionary, CC BY-SA 4.0.
smurf attack
Today it means: A largely obsolete form of denial-of-service attack, involving broadcasting ICMP packets with the intended victim's IP address to a large computer network, causing the victim's computer to be overwhelmed by the response packets from the network. Full definition →