Interestingly, in My Bonny Lad it is Ower Land rather than Ower Lang, thus echoing the original typo, but getting further away from the meaning which Gnomad suggests is the same as in BOM which is obviously isn't, because then you get he's gone ower lang with a stick in his hand which makes no sense and loses the rhyme. Otherwise ower lang in thy bed is about laziness, or rather perceived laziness, or maybe something more picturesque perhaps? Assuming the lad & lass aren't actually related of course, or maybe they are - kissing cousins in the Northumbrian wilds, keenly (and cannily) copulating morn and night in their one room hovel whilst the grandmother (who'll only be in her early thirties, but feeling pretty old even so) looks after her baby grandchild. Grim stuff...