"Fier" (also spelled "pheare" etc.), according to the OED, could mean a mate of either sex, *or* a spouse. It appears in a terrific speech, probably written by Shakespeare, in _The Two Noble Kinsman_.

Speaking of Burns, one might note in The Jolly Beggars,

With the ready trick and fable,
Round we wander all the day,
And at night in barn and stable
Hug our doxies in the hay.

A fig for those by law protected! etc.