Not really a slang term (that I am aware of). Just humourous because of the direct translation. whichi is "octopus balls"
A famous Osaka, Japan dumpling made of batter, octopus, tenkasu, pickled ginger, konnyaku, and green
onion, topped with okonomiyaki sauce,
green laver (nori), mayonnaise, and katsuobushi (fish shavings). Making takoyaki requires a takoyakiki, a special frying
pan made of cast iron with hemispherical molds.A piece of octopus piece is in the middle of a round flour paste ball which is not
100%
cooked.
The direct translation is "octopus balls"
The size of takoyaki are about a mouthful, and nothing more.