Dave, the word "nouriss" probably also refers to a nursing mother, i.e. breastfeeding, so in that sense she was not the baby's wet nurse but in fact the baby's mother.

The line "little ken I my bairnie's father" (in one version of the song) would indicate that it is her own baby.

The word nouriss is derived from the Latin word nutrire meaning to feed or nourish

nourisher (English)

"nourish (English)

"Origin & history

"From Middle English, from Old French nouriss-, stem of one of the conjugated forms of norrir, from Latin nutrire ('to suckle, feed, foster, nourish, cherish, preserve, support')."