This word is a Scots variant of 'joy', and can mean a sweetheart or lover, or be a term of endearment akin to 'dear' or 'darling'. Probably the best-known example of it is in the Burns song John Anderson My Jo (1789), and there are many other examples of it in the Dictionary of the Scots Language, dating from the 16th century onwards. A jo can be either male or female, and can be anyone dear to the speaker – not necessarily a lover. Both these points are illustrated by this quotation from David Lyndsay's Ane Satyre of the Thrie Estaitis (1540): “Quhat wald thow, my deir dochter Jenny. Jenney my joe, quhat dois thy daddy?”  

Jo also appears in phrases such as play ‘jook my jo' meaning to flirt, as in S R Crockett's The Raiders (1894): “That's what auld Airlie gies to young birkies like you that come in graund coats to play ‘Jook my jo' wi' his lasses.” Then there's penny-jo, meaning a prostitute. R L Stevenson in Underwoods (1887) refers unglamorously to “Penny joes on causey stanes” in a list of unfortunates, sandwiched between “hoastin weans” and “auld folk wi' the crazy banes”.
Jo also appears in the expression ‘hinny [honey] and jo'as in this example from P H Hunter's James Inwick (1894): “it hadna been a' hinny an' jo wi' us” indicating the variable course of true love.
But most of the examples in the dictionary show the joy that the word expresses. From the animal world, we have the wholehearted “sang o the lintie a-courting his joe” in The Poets and Poetry of Linlithgowshire, ed. A M Bisset (1896), and the euphoria of love is summed up in Violet Jacob's Bonnie Joann (1921): “But my feet danced oot to meet my joe”.
Scots Word of the Week is written by Ann Ferguson of Scottish Language Dictionaries