This expression originates from the way an octopus is stretched out to dry. All eight legs are spread out and stretched to their limits. Thus, when the pretty young starlet or the renowned architect is suddenly in great demand, the impression is like a hippari dako.
(Style: A=informal/male, B=formal/male)
A: Ano shinjin kashu, saikin zuibun ninki ga dete kita ne.
B: Soo na n desu yo. Moo iroirona tokoro kara hippari dako de ne. Uchi no zasshi no intabyuu toru no mo taihenna kurai na n desu yo.
A: That new singer is really getting popular.
B: She sure is! She’s in great demand; actually it was difficult booking her for our magazine interview.