It means "unreal"

Usage examples of syn.

But syns it is so that nowe in our dayes ar so many neglygent and folysshe peple that they ar almost innumerable whiche despisynge the loue of vertue: folowe the blyndenes and vanyte of this worlde: it was expedient that of newe some lettred man, wyse, and subtil of wyt shulde awake and touche ye open vices of foles that now lyue: and blame theyr abhomynable lyfe.

Now, the holding of land upon the hills gave to the Syns, as it did to other Marshmen in like case, a sense of security, for the reclaimed pasturage of Romney Marsh owed its existence to the Dymchurch Wall, which held the sea in check.