Also spelt Allardyce. <ref>David Dorward, Dictionary of Scottish Surnames, 4</ref> Black states that this is not a very common name, 'but all who hold it believe in their descent from the old family which was settled on the banks of the Bervie water'. <ref>George Way of Plean and Romilly Squire, Scottish Clan and Family Encyclopedia, 351</ref> The ancient barony of Allardice, sometimes styled 'Alrethes' lies in Kincardineshire near the parish of Arbuthnott. <ref>Plean, Squire, Encyclopedia</ref> Some element of the name is probably the Gaelic word 'deas' -meaning 'south'. <ref>Dorward, Dictionary, 5</ref> Nisbet states that William the Lion granted a charter to the family of the lands of Allrethis, and since that time the surname of the family has been taken from those lands.<ref>Plean, Squire, Encyclopedia, 352</ref>
Thomas Allardice of that Ilk received lands from David II. <ref>Plean, Squire, Encyclopedia, 352</ref> Alexander de Allyrdas who witnessed a charter of the lands of Glack around 1294 is likely the same Alisaundre de Allerdashe of the county of Kinkardyn who rendered homage in 1296. Walter de Allerdas also rendered homage in the same year. <ref>http://www.scotclans.com/scottish_clans/clan_allardice/history.html</ref> Sir James Allardice was Clerk of the King's Treasury and Archdeacon of Moray around 1478.<ref>Plean, Squire, Encyclopedia, 352</ref>
In 1662 Sir John Allardice of that Ilk, described by Anderson, as 'Chief of that ancient family', married Mary, sister of the Earl of Menteith. This line ended when the last heir of Allardice, Sarah Allan, married Robert Barclay of Urie, MP for Kincardineshire in 1776. <ref>Plean, Squire, Encyclopedia, 352</ref>
Their granddaughter, Margaret Barclay Allardice, claimed, by descent from her Allardice blood, the earldoms of Strathearn, Monteith and Airth, as last of the line descended from Prince David, son of Robert II. <ref>Plean, Squire, Encyclopedia, 352</ref> No decision was however reached on these titles and they remain dormant. <ref>Dorward, Surnames, 5</ref>
She assumed by royal licence of 2 July 1883 the name and arms of Barclay-Allardice. The family continued to prosper, having taken up residence in England, but the present successors to this noble line now use the name of Barclay alone. <ref>Plean, Squire, Encyclopedia, 352</ref> The chiefly family were therefore absorbed through the marriage of an heiress into Clan Barclay.