One's native land, or the native land of one's ancestors.
The most common use of this expression in English derives from the early European settlers to the USA. Two prominent figures of the Pilgrim Father settlers were John Robinson and William Brewster. While in Holland, and planning the emigration to America on the Mayflower, they wrote to Sir Edwyn Sandys in 1617:
"We are well weaned from ye delicate milke of our mother countrie, and enured to ye difficulties of a strange and hard land, which yet in a great parte we have by patience overcome."
In his style of naming his children, Brewster followed, and given the early date we might better say pioneered, the Pilgrim tradition of choosing names in accordance with biblical themes. His first child was called Jonathan, but he soon got into his stride with Patience, Fear, Love and finally, Wrestling.
The 'mother country' to Brewster and Robinson was, of course, England and that was generally what was meant when the phrase came into use in the USA. They didn't coin the phrase themselves but probably read it in the works of a prominent Puritan of the day - Arthur Golding. Brewster in particular was well read and owned a library of some 400 books in Latin and English and was sure to have read Golding's well-known translation of Ovid's Metamorphosis, 1567:
"They went too Phebus Oracle, which willed them too go Untoo theyr moother countrey and the coastes theyr stocke came fro."
In Ovid's Latin original the phrase appears as 'antiquam matrem'.
It is perhaps an indication of the way of thinking of the Pilgrim Fathers that they chose to adopt the term 'mother country', as opposed to 'fatherland', which was used by others in the 17th century to denote the country of one's heritage.
The phrase was coined before America was settled by English speakers and popularised by two Englishmen living at the time in Holland. Nevertheless, 'mother country' can be said to be the first English phrase to have a strong American association - some might say, the first American phrase.
See other phrases that were coined in the USA.