E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Skains-mate or Skeins-mate.
A dagger-comrade; a fencing-school companion; a fellow cut-throat. Skain is an Irish knife, similar to the American bowie-knife. Swift, describing an Irish feast, says, A cubit at least the length of their skains. Green, in his Quip for an Upstart Courtier, speaks of an ill-favoured knave, who wore by his side a skane, like a brewers bung-knife.
Scurvy knave! I am none of his skainsmates.Shakespeare: Romeo and Juliet, ii. 4.