The unglossed headword adjective probably refers to the city of this name, in the NE Peloponnese (cf. e.g. epsiloniota 337, mu 141, nu 218); but see also sigma 402.
[1] The phrase "Sikyonian plain" occurs twice in Lucian (Ikaromenippos 18.3; Navigium 20.13).
[2] (Addendum lacking, Adler reports, in mss AF.) Same proverb in the paroemiographer Macarius Chrysocephalus (C14 CE) 7.62, where it is explicated as "in reference to those doing something daring but accomplishing nothing". He follows this in 7.63 with "Sikyonian runaway", meaning the same. One wonders how and why, in late antiquity, Sikyonians acquired such an unenviable reputation.