The headword phrase comes from Thucydides 2.94.1 (web address 1). It occurs during his description of an episode in late summer 429 BCE, when, before disbanding for the winter, Spartan/Peloponnesian naval forces try a surprise attack on Athens' harbor, Peiraieus (pi 1455) -- at that time rather poorly defended -- via the island of Salamis (cf. sigma 49). At this stage of the narrative Thuc. has just commented that the residents of Athens town supposed (incorrectly) that Peiraieus had already been breached; he now says that those in Peiraieus itself thought (correctly) that the enemy had seized Salamis and were thus on the point of sailing into Peiraieus.
The Suda, unfortunately, mangles Thucydides' idiom here. As Adler notes, Thucydides' text reads not the untranslatable o(\s ou)k but o(/son ou)k, for which cf. omicron 700.
[1] From the scholia to the aforementioned passage.