The headword is the aorist active participle, neuter accusative singular, of the verb paratugxa/nw, I happen to be present, available; see generally LSJ s.v. It is extracted from Thucydides 1.76.2 (web address 1), where Athenian ambassadors in Sparta in late 432 BCE explain Athens' post-Persian-War acquisition of an empire as taking natural human advantage of what simply happened to be within their power.
[1] From the scholia to the aforementioned passage in Thucydides. The point, made both directly and with a parallel, is that an accusative absolute is being used rather than a genitive absolute (see Smyth §2076, web address 2). For e)/con (of which there are three instances in Thucydides), see also epsilon 1799, epsilon 1800.