This entry reproduces Photius pi1131 Theodoridis). The headword phrase itself occurs in, and is very probably quoted from, Plato, Theaetetus 142A (web address 1): "I wasn't really in the city." -- "Well where [were you]?" -- "As I was going down to the harbour I met Theaetetus...". See on the passage J.D. Denniston, The Greek Particles (Oxford 1954) 333 ('Following the rejection of a supposition').
On this usage of mh/n see generally LSJ s.v., III.